UBV OBSERVATIONS OF THE SYMBIOTIC NOVA HM SAGITTAE

A homogeneous catalog of UBV observations of the symbiotic nova HM Sge over the period 1979 - 1993 was compiled. The mean brightness of the star in B and V was constant until 1982; then, it began to monotonically decline (the rate of brightness change in B was 1.4 x 10(-4) per day). A systematic decrease in brightness in the U band has been observed since 1986. The effect of bright bands in UBV in 1977 and 1987 was assessed. With this effect in mind, the optical continuum of the star in 1987 is shown to have been purely nebular with an electron temperature of about 15 000 K. The photometric behavior of HM Sge after the outburst bears a strong resemblance to that of the other symbiotic nova V1016 Cyg. An analysis of the track of HM Sge in a two-color diagram U-B, B-V revealed a drop in electron temperature in its gaseous envelope after the outburst. The track of the star in color-magnitude diagrams suggests that the contribution of the hot star is reduced with time. A search for periodicity in the brightness variations in B after the outburst consistent with the pulsation period of the Mira yielded a negative result. The brightness measurements of HM Sge before the outburst similarly revealed no periodicity; however, the maximum light of the object was observed near the maximum phase of the Mira, as after the outburst.