EB 3 Lyrae stars

- These are eclipsing systems having ellipsoidal components and light curves for which it is impossible to specify the exact times of onset and end of eclipses because of a continuous change of a system's apparent combined brightness between eclipses; secondary minimum is observed in all cases, its depth usually being considerably smaller than that of the primary minimum; periods are mainly longer than 1 day. The components generally belong to early spectral types (B-A). Light amplitudes are usually less than 2"' in V. GCVS

The f) Lyrae stars (EB) are another subgroup of eclipsing binaries segregated according to light curve shape (Figure 7.4). The light curve varies continuously between eclipses, making it difficult to specify the moments of the beginning and the end of the eclipses. To distinguish between EBs and EWs, according to the GCVS, the former generally have primary and secondary eclipses significantly different in depth, orbital periods longer than a day, and spectral types B or A.

Light curves of this shape are supposed to be produced by an eclipsing binary in which one or both components is highly ellipsoidal. One of the components may even fill its Roche lobe.

Among the EBs one may find binaries of very different evolutionary status:

(a) unevolved binaries consisting of two main sequence stars but a relatively short orbital period, with XY UMa an example;



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F-gure 7.4. Ligh, of the EB^ypevo,,^ t> Lyr. Dotap^

by the HIPPARCOS mj«J

Used with permission

(b) binaries in which one or both components is evolved but not yet filling the Roche lob, with f And an example;

(c) semidetached binaries undergoing mass transfer from the evolved to the unevolved star, with fi Lyr an example;

(d) binaries with one star highly evolved, a hot subdwarf or a white dwarf, and the other, producing the ellipticity effects, with AP Psc an example.

Ironically, some binaries classified as EB are not eclipsing at all. The light variation is produced entirely by the ellipticity effect and the two minima are unequal as a result of greater limb-darkening effects on the pointed end of the highly distorted star.

The first EB discovered, and the prototype of the group, was fi Lyrae. The same John Goodricke of Algol fame discovered the variability of ft Lyr one year later, in 1784. fi Lyr is extremely complex and interesting. The brighter star fills its Roche lobe and is transferring matter onto the other star so rapidly that a thick (both optically and geometrically) disk has built up which almost completely obscures the underlying mass-gaining star itself. This mass transfer causes the orbital period to increase at a furious rate. In the 210 years since Goodricke's 1784 timing, the period has increased from 12^8925 to 12,?93854, an increase of 0.35%.

Observing fi Lyrae stars can be an exciting project for visual observers and many EB variables fall within the visual observing capabilities of amateur astronomers. For those wishing to embrace a deeper challenge, hundreds of EB variables exist that must be examined using photometric methods. In either case, observing these interesting stars will provide a lifetime of enjoyment.

Binary Eclipsing Systems


Key fa Mixed stars

Mixed amplitudes

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