MT Ser is the binary central star of the planetary nebula Abell 41.
Its double star nature manifests itself through orbital variations with
a period of
discovered by Grauer & Bond
(1983) (hereafter referred to as GB83). Green et al. (1984)
classified the primary as a sdO star. The secondary is suspected by GB83 to be
a late type dwarf star, but - as Green et al. (1984) pointed
out - an alternative model is not excluded. GB83 showed
that angular momentum loss from the system caused by gravitational
radiation can bring the surface of the secondary in touch with its
critical Roche surface within the relatively short time of
years within their model.
When this happens the sub-dwarf primary
will have evolved into a white dwarf and MT Ser is expected to
initiate activity as a cataclysmic variable. It should therefore presently be
regarded as a pre-cataclysmic binary.
Such systems are thought to be the result of common envelope evolution of an initially much wider binary: On the red giant or asymptotic giant branch the more massive primary expands to an extent that it engulfs the low mass secondary which is still on the main sequence. Frictional angular momentum loss lets the secondary spiral towards the core of the giant. Depending on the initial parameters and the details of the evolution during the common envelope phase, after the ejection of the envelope the core of the giant - now a hot sub-dwarf - and the late type dwarf may reappear as a close binary with an orbital period of the order of hours.
Not many binaries in the phase between the common envelope and the birth as a cataclysmic variable are known. MT Ser as the central star of a planetary nebula has obviously emerged from the common envelope phase only quite recently. Therefore, the system might still exhibit features which can shed some light on the so far largely unknown details of the common envelope evolution. Although this is not the topic of the present work a basic understanding of the structure and the dynamics of MT Ser are an obvious prerequisite for an investigation of such features. Since these fundamental parameters are still unknown or at least quite uncertain, we will try here to improve this situation.
Although the planetary nebula around MT Ser, Abell 41, has been the subject of numerous studies which sometimes included global properties of the central star (Abell 1966; Shaw & Kaler 1989; Tylenda et al. 1989; Tylenda et al. 1991; and Pollacco & Bell 1997; to cite only a few of the more important papers) detailed investigations remain scarce. In their discovery paper of the orbital variations GB83 derive some limits of system parameters but were not able to settle down on definite values. Not even the orbital period is certain: It cannot be excluded that the period is twice as long as the value quoted above, a suspicion which will be substantiated in the present paper. Taking the results of GB83 together with a spectrum in the blue spectral range, which apart from the strong nebular emission lines also reveals some absorptions of the central star, permitting it to be classified as sdO, Green et al. (1984) modified the model for MT Ser somewhat (in particular concerning the temperature of the hot component) but otherwise largely agreed with GB83. However, they also discuss briefly an alternative model consisting of two hot sub-dwarfs. Further detailed observations of MT Ser have not been published.
Here, we report about time resolved photometric observations of the system with the aim to obtain a high quality orbital light curve which enables the determination of reliable system parameters via light curve synthesis using the Wilson-Devinney code (Wilson & Devinney 1971; Wilson 1979).
Copyright ESO 2001