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The star belongs to the zero-age main sequence (ZAMS) and is much smaller in size than the critical RL. The duration of the first stage is set equal to the hydrogen burning time, , which is a known function of the stellar mass only (Iben and Tutukov, 1987):
The radius of the ZAMS star is
and its luminosity is
The initial mass of the primary and mode of the first mass exchange (which is determined by the initial orbital period and masses of the components; see Webbink, 1979) determine the mass and type of core that will be formed during stage I. For example, for single stars and primaries in type C binaries that fill their Roche lobes having degenerate cores, we use the expressions
where is the maximum mass the star attained during its previous evolution and is the Chandrasekhar limit for WD mass.
A MS-star that accretes matter during the first mass transfer is considered to be a rapidly rotating Be-star that is distinguished by its different stellar wind rate (see below).