IT IS 4: 32 P. M. on Aug. 18, 1991. At the Crimean dacha where Mikhail Gorbachev has retreated for a summer break, an emergency signal warns the duty officers minding the Soviet Union's nuclear button that they have been cut off. In their flat 100 meters from the Gorbachev dacha, they watch their television screen die, along with all other means of communication to the president or to Moscow. In an instant, Gorbachev had lost all power to launch a nuclear strike.


Generals apparently loyal to the coup plotters had cut all connections with the president's dacha at Foros, including those to the three duty officers guarding his "nuclear briefcase". In it were the special codes needed to fire atomic weapons.


Americans call it the "football" and Russians the "chemodanchik", or briefcase, but they serve the same purpose. Both comprise a 24-hour-a-day, 365-day-a-year communications service whereby the supreme commanders of the superpowers control their nuclear arsenals.


At 4: 32 P. M. , when the duty officers were isolated, Gorbachev's nuclear button was disconnected; it was not restored until the coup had failed and he had returned to Moscow in the small hours of Aug. 22.


In the interim, could the Soviet Union have launched a nuclear strike?


Russia's public prosecutor, Valentin Stepankov, who is investigating the coup plotters, has since made it clear that Moscow was not left defenseless at all.


Stepankov recently published in Nezavisimaya Gazeta an excerpt of interviews with army officers involved. According to their testimony, when Gorbachev was switched off there was barely a ripple of concern back at the Operational Division of the General Staff in Moscow, the nerve center for the Soviet nuclear deterrent.


It turns out that approval from the country's paramount political leader had not been necessary for launching a strike. The briefcase was nonessential.


"Control of the nuclear forces was in the hands of the supreme generals of the army and the KGB", Stepankov concluded. "Such is the true value of the 'briefcase'".


That was then. and since August 1991, the Soviet military has broken into numerous republican armies, political structures are in a state of evolution and in the Kremlin, the status of the top leadership remains fragile.


Stepankov's investigation of the secretive world of Soviet nuclear operations was revealing about what happened during the coup. But it stopped short of answering one crucial question.


Whose finger is on the nuclear button now?


THE BRIEFCASE with the codes, at least, appears to have passed directly from Mikhail Gorbachev to Boris Yeltsin on Christmas Day. Soon afterwards it was agreed that Russia should be the only inheritor of the Soviet nuclear arsenal.


But according to Western specialists, the question of whose finger, in fact, controls the Russian nuclear button remains as difficult to answer as it was during the Soviet era.


What the coup proved -- that the owner of the nuclear briefcase does not necessarily control Russia's nuclear power -- Western analysts have always suspected. They believed then that there were two forms of control over the nuclear arsenal, and they now say there is no reason to think that this has changed.


The first is known as "authorizing" controls, of which the briefcase is one. By this mechanism, Yeltsin is able to send a coded message to the Operational Division of the General Staff, authorizing a nuclear strike.


The codes are complicated. They can order an immediate strike, for instance, or a conditional one based on the fact, to be verified, that missiles are coming the other way.


According to Stepankov, before the collapse of the Soviet Union at least three people had identical briefcases: Gorbachev, who was also general secretary of the Communist Party; the defense minister, Marshal Dmitry Yazov, and the chief of the General Staff, Gen. Mikhail Moiseyev. All three could use the codes to authorize the use of nuclear weapons.


Moiseyev assured the Italian daily Corriera del Sera in an interview soon after the coup that he had changed the codes while Gorbachev was out of action, making all three briefcases useless and ensuring that if anybody did get hold of Gorbachev's codes, they would be harmless. But it appears that having the authorizing codes is neither necessary nor sufficient to launch a strike. In this sense, the country's military commanders have more power than its political leaders, up to and including the president.


The second set of controls are "enabling". These are codes that make up links in a chain of command that actually permits missiles to be fired. They have to be typed into control systems, in some cases at the launch site, in order for the missiles to be fired. The difference between the two forms of control, "authorizing" and "enabling", is rather like that between the legal right of ownership and possession. Both are required if you are to enjoy all the fruits of ownership. But if you lack possession, you have nothing. An analyst who has studied the Soviet nuclear arsenal in the West, and who declined to be identified, put it this way: "Most of these systems can physically be enabled without any need for authorization". If the military generals do not choose to wait for authorization from the president, they do not need to.


SO WHEN Viktor Boldyrev, the duty officer of the 9th Department of the General Staff in Moscow, discovered after 5 P. M. that communications with Gorbachev's briefcase had been cut off, he did not bat an eyelid, according to Stepankov's account.


It was already half an hour since the lines went down. He asked someone to look into it and went home as usual. The loss of communications with Gorbachev, after all, had no bearing on the country's ability to launch a nuclear strike.


It was 7: 45 the following morning, on Aug. 19, before Boldyrev was told the reason for the communications breakdown with Foros -- "a landslide". Having heard on the radio about the coup, he drew his own conclusions. But still, there was no panic. Only at 8 A. M. did Boldyrev begin attempts to reconnect Gorbachev, he told Stepankov.


A deputy head of the KGB eventually ordered communications with Foros to be restored. It was a KGB general named Generalov who ordered the briefcase officers in Foros to stay put and keep quiet. It was the KGB who controlled entry and exit to the dacha and prevented the relief watch from entering the perimeter.


And at the nuclear nerve center in the General Staff, there was no cause for concern. As General Moiseyev told Corriera del Sera, he was in complete control of the nuclear arsenal, having blocked the politicians out of the system.


TODAY, TOO, it appears that the chief of the General Staff, Colonel General Viktor Dubynin, could have ultimate control. He drives the so-called "hedgehog", a Zil limousine that looks like a pin cushion for antennae. It ensures that he is always in contact with the nuclear command.


Yet it is impossible to say for sure how the system works. The chain of command for launching a nuclear strike was a closely guarded secret in the Soviet Union; in today's Russia, it is no less secret, and the state of flux of the political institutions adds an additional element of uncertainty.


One big unknown is the role of the former KGB, now divided into the Security Ministry for domestic affairs and Russian Intelligence for operations abroad. The Security Ministry, in many ways, appears to be carrying on the work of its defunct predecessor.


Under Gorbachev, ultimate nuclear authority probably rested with the Defense Council, technically a branch of the Supreme , Soviet, but in fact drawn from the Politburo. In wartime, the Council would have expanded to form the staff of the supreme high command.


But when the Soviet center disappeared, so did the Politburo and the Defense Council. Not until July this year was a replacement formed that could fill the vacuum, namely Yeltsin's increasingly powerful Security Council.


The Security Council is run by its secretary, Yury Skokov, a former defense factory manager. It is headed by Yeltsin and includes Defense Minister Pavel Grachev; Security Minister Viktor Barannikov; Vice President Alexander Rutskoi; the first deputy chairman of parliament, Sergei Filatov, and acting Prime Minister Yegor Gaidar.


Its Workings are secretive, and Skokov does not give interviews. The Defense Ministry declined to answer questions for this article, saying that the subject was "not for the press".


One can only surmise, as an American study of the former Defense Council did, that in an emergency, the Security Council "presumably has the exclusive authority to approve the use of nuclear weapons".


There is in America, too, concern that the best laid command and control systems have been made ineffective by the march of nuclear technology, which can now launch a nuclear strike in a matter of minutes.


As Alexei Arbatov, director of the Weapons Control Center, said in an article for Nezavisimaya Gazeta, the president is effectively "no more than a well-trained monkey that responds to a signal and pulls a lever".


In today's unstable Russia, where the possibility of a KGB or military takeover is not entirely impossible, it seems pressing to find out who, if not the president, really controls the button. The chief of the General Staff, above all, wields the enabling controls and is doubtless loyal. But the lesson of Stepankov's investigation is that maverick generals in the army and the KGB, which has a presence throughout the armed forces and particularly in the Strategic Rocket Command, were able to turn Gorbachev's button on and off at will.


Who can be sure that no one will choose to switch off President Yeltsin?