In order to avoid influence of the late-type component, period changes of binaries in which both stars are of early spectral types must be analysed. COL is supposed to appear at F5 in single stars. Since losers in binaries undergoing mass outflow may develop deeper COL than single stars of the same spectral type we will restrict to binaries with both stars earlier than F0.
The following analysis will be devoted to the Algol-type semi-detached systems because we can make use some of their suitable features: (a) their status is forecasted by models and they represent a rather well understood epoch of evolution of close binaries; (b) mass transfer in this binary can occur only in one well defined sense, i.e. from the Roche-lobe filling component (loser); (c) in principle, the flowing matter can be observed since the gainer may be well inside its Roche lobe; (d) the mass transfer or loss rate can be determined from spectroscopy or photometry independently of the analysis of changes of the orbital period in some cases.
The early-type binaries which are a matter of this study contain high-mass stars. Since these stars evolve more rapidly than those with smaller masses we can also expect more pronounced evidences for mass transfer events and evolutionary processes. Although the available timings cover a limited interval (usually a century or shorter) which may be too short for the convincing changes produced by the mass transfer to manifest themselves even in the high-mass binaries this handicap can be at least partly overcome by simultaneous analysis of a set of systems and by a search for common features. The activity of the cool component is supposed to be responsible for most period changes in Algols. The O-C variations in the early-type binaries which should be free of this activity could then allow for a better insight to the importance of the mass transfer for the period changes, especially in those cases where an independent derivation of the mass transfer rate from the photometric or spectroscopic activity is possible.
Nevertheless, some reasons for caution should be noted. The number of the observable high-mass eclipsing binaries with an early-type star filling its Roche lobe (hereafter ET-systems) is significantly smaller than that of Algols with late-type losers (hereafter LL-Algols). This also implies a small number of ET-systems with the well determined parameters. ET-systems also generally display shallower eclipses because the luminosities of both components are more similar to each other than in LL-Algols. Accurate timings are therefore more difficult especially for visual observers.
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