OPTICAL OBSERVATIONS OF 4U-1915-05, AND THE STABILITY OF THE LONG-TERM EPHEMERIS

We present the results of a comprehensive campaign of optical photomet ry of the dipping and bursting Low Mass X-ray Binary 4U 1915-05 (herea fter 1915-05). During 1990 September we monitored 1915-05 while the sy stem was in an anomalously faint optical state. We find clear evidence for the presence of both the X-ray and optical periods in the data, a lthough at phases which preclude a simple interpretation on the basis of standard models. Optical dips at phase similar to 0.5 were also obs erved. We show that the intrinsic luminosity of the disk may well make a significant contribution to the overall optical flux of the system, increasing the detectability of any SU UMa-like, precessing behavior. We also discuss our long term, 7 year database of 1915-05 photometry, and show that the mean optical modulation can be apparently described by a simple ephemeris over this timescale. This suggests that the opt ical modulation indeed represents the binary period: however, further observations are still required to confirm or otherwise the proposed l ong term phase stability.