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It is known that binary star components containing red giant can under go mass transfer when their radii exceeds the Roche limt. Do main sequence binary stars also undergo mass transfer by this mechanism?

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    $\begingroup$ Do you mean two main sequence stars in a binary system? $\endgroup$ – Rob Jeffries Aug 21 at 6:03
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    $\begingroup$ yes. when both stars are main sequence stars $\endgroup$ – Jiswin Aug 21 at 9:43
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Yes. An example would be the Cataclysmic Variable stars (CVs), where the donor star (often in the main sequence), loses material via Roche Love overflow onto a compact white dwarf companion.

The other obvious example (though less common) are the Low Mass X-ray Binaries (LMXBs), where the Roche Love overflow is onto a neutron star or black hole.

Nelson & Eggleton (2000) did a theoretical study of Case A mass transfer (where the donor is hydrogen burning star) and found several subcategories where all the mass transfer takes place whilst both stars are on the main sequence (labelled AD, AR and AS). In all three cases, the endpoint is a contact binary of the W UMa type, where both stars fill their respective Roche lobes.

W UMa stars are very common, but that is because their lives are long, whereas the period over which mass is transferred between the progenitors is very short by comparison, so objects "caught in the act" will be rare. The basic picture is that they start out as short-period, deteached binary systems. But as they evolve, two things happen. (i) If they are of low-mass then dynamo generated magnetic activity can result in angular momentum loss which brings them closer. (ii) Normal stellar evolution on the main sequence leads the stars to get bigger. The two combine and lead to the primary star filling its Roche lobe, transfering mass and accelerating the process towards a W UMa status.

The exception is the AS case, where the transfer occurs on the "nuclear timescale" (e.g. the Sun's nuclear timescale is 10 billion years) and the mass transfer is very slow.

Thus semi-detached main sequence systems are either rare or not doing much, which makes them hard to spot. Nevertheless, they are found if you look in the literature. e.g. Deb & Singh (2011) identify several in a sample of contact or close-to-contact eclipsing binaries, from their distinctive light curve shapes. Other authors refer to these as Pre-Contact W UMa Binaries (PCWBs) and have catalogued some tens of objects (Samec et al. 2013).

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    $\begingroup$ A classic example is the naked-eye star Beta Lyrae en.wikipedia.org/wiki/Beta_Lyrae. $\endgroup$ – John Doty Aug 21 at 15:28
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    $\begingroup$ Oh, yes, I'd forgotten Algol. In the mid '70s I was in the SAS-3 science ops center when it popped up unexpectedly in an x-ray modulation collimator correlation map. How little we knew in those days... $\endgroup$ – John Doty Aug 21 at 16:40
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    $\begingroup$ @JohnDoty Whoops, Algol is of course Beta Per. not Beta Lyra. However the same comment applies to Beta Lyr - the mass transferring star has left the main sequence. $\endgroup$ – Rob Jeffries Aug 21 at 17:01

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