An analysis of the light curve of the post common envelope binary MT Serpentis

Photometric observations of MT Ser, the central star of the planetary nebul a Abell 41 are presented. The periodic modulations detected by Grauer & Bon d (1983) are confirmed, thus firmly establishing the binary nature of MT Se r. The significantly enlarged time base permits us to derive more accurate ephemeris. The orbital period is either P-1 = 0.113226533 days or twice tha t value, P-2 = 0.226453066 days. We analyze the light curve (after a carefu l subtraction of the nebular contribution) with the Wilson-Devinney light c urve synthesis routine. Since it is not a priori clear which is the true or bital period of MT Ser, two radically different models, one based on P-1 th e other on P-2 are considered: (1) A low temperature component orbiting aro und a hot sub-dwarf. The variability is then due to a reflection effect tog ether with ellipsoidal variations of one or both components. (2) Two hot su b-dwarfs of similar temperature and luminosity, partially eclipsing each ot her and exhibiting ellipsoidal variations. In both models, the primary as w ell as the secondary component are required to almost fill their respective Roche lobes. A contact configuration is possible. Pros and cons can be fou nd for either of the two models. A final decision between them has to await the observations of a radial velocity curve. The orbital period is current ly decreasing at a rate of (P) over dot/P = -1.15 x 10(-9) d(-1). Interpret ing this as due to mass loss via a stellar winds permits us to estimate mas s loss rates depending on the different model assumptions.