2001A&A...377..898B

Photometric observations of MT Ser, the central star of the planetary nebula Abell 41 are presented. The periodic modulations detected by Grauer & Bond (1983ApJ...271..259G) are confirmed, thus firmly establishing the binary nature of MT Ser. The significantly enlarged time base permits us to derive more accurate ephemeris. The orbital period is either P₁=0.113226533 days or twice that value, P₂=0.226453066 days. We analyze the light curve (after a careful subtraction of the nebular contribution) with the Wilson-Devinney light curve synthesis routine. Since it is not a priori clear which is the true orbital period of MT Ser, two radically different models, one based on P₁ the other on P₂ are considered: (1) A low temperature component orbiting around a hot sub-dwarf. The variability is then due to a reflection effect together with ellipsoidal variations of one or both components. (2) Two hot sub-dwarfs of similar temperature and luminosity, partially eclipsing each other and exhibiting ellipsoidal variations. In both models, the primary as well as the secondary component are required to almost fill their respective Roche lobes. A contact configuration is possible. Pros and cons can be found 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 currently decreasing at a rate of {dot}(P)/P=-1.15x10^–9d^–1. Interpreting this as due to mass loss via a stellar winds permits us to estimate mass loss rates depending on the different model assumptions.