1. Detection of nonthermal optical flares in low mass X-ray binaries. Eleven low-mass X-ray binaries have been observed with the MANIA (Multichannel Analysis of Nanoseconds Intensity Alteration) complex; X0420+32, A0620-00, X1728–169, X1813–14, X1957+11 in 1986 at the 6 m telescope of SAO , and 2S0921–630, 4U1543–475, 4U1636–536, 4U1559–487, MXB1735–44, 4U1822–371 in 1991 at the 2.15 m telescope of CASLEO (Argentina) . Analysis of the data using the special y
2 and d
2 function formalism ,  has shown no brightness variations on time-scales of 10−7... 10 s for all objects except A0620–00 and MXB1735–44.
A0620–00. The five millisecond flashes were detected on 1986 February 13 (Fig. 1). The first two flares have durations of 3 ms and 5 ms and their rise times were 1...2 ms. The other three events lasted 0.4…0.5 ms with rise times of about 0.1 ms. The lower limit of the flare amplitudes is 40 counts ms−1 and the corresponding brightness temperatures are 5 109 K for the first two and 5 1011 K for the others.