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 monotonical ly decline (the rate of brightness change in B was 1.4 x 10(-4) per da y). A systematic decrease in brightness in the U band has been observe d since 1986. The effect of bright bands in UBV in 1977 and 1987 was a ssessed. With this effect in mind, the optical continuum of the star i n 1987 is shown to have been purely nebular with an electron temperatu re of about 15 000 K. The photometric behavior of HM Sge after the out burst bears a strong resemblance to that of the other symbiotic nova V 1016 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 o utburst consistent with the pulsation period of the Mira yielded a neg ative result. The brightness measurements of HM Sge before the outburs t similarly revealed no periodicity; however, the maximum light of the object was observed near the maximum phase of the Mira, as after the outburst.