1 Introduction

One of the most intensely studied binary systems is the bright eclipsing binary Algol (=$\beta$ Per). Already the Chinese noted Algol's brightness changes, but it was Goodricke (1783), who rediscovered its brightness changes, noted their periodicity, and provided the essentially correct interpretation by attributing the brightness change to a second, darker body in the system. In modern language Algol is an eclipsing photometric and spectroscopic binary with well determined system parameters. According to the recent determination of system parameters by Al-Naimiy et al. (1985), Algol contains a B8V primary ( $M = 3.77~M_{\odot}$, $R = 3.07~R_{\odot}$) and a K2III secondary ( $M = 0.82~M_{\odot}$, $R = 3.27~R_{\odot}$). Thus the two stars have almost the same size, but their masses differ by almost a factor of five. The more evolved secondary is lighter than the less evolved primary in seeming conflict with stellar evolution theory which predicts faster evolution for more massive stars. This conflict can only be solved by assuming substantial mass transfer from the present-day secondary to the present-day primary, a suggestion first made by Crawford (1955). Plenty of evidence now exists for mass transfer in Algol-type binaries in the form of gas streams, disk- and related features. The role of mass loss in the evolution of Algol systems is presently not clear; in addition to significant mass being donated to the present-day primary, significant mass (and angular momentum) may be lost from the system due to a wind.

Stellar evolution theory predicts that in stars more massive than $\approx$2 $M_{\odot}$ the CNO-cycle is the dominant mode of energy generation. In the CNO-cycle the atoms of carbon, nitrogen, and oxygen are used as catalysts to convert hydrogen into helium. Its nuclear equilibrium is such that most of the catalytic nuclei participating in the cycle at any given time are found in the form N¹⁴. Therefore one expects to find substantial nitrogen enrichment if the cycle is interrupted by lowering the temperature and CNO-cycled material is subsequently brought up to the surface. Mass transfer in a close binary as in an Algol system is a rather dramatic way to expose nuclearly processed material at the surface. Less dramatic ways to achieve the same result also occur during the regular course of stellar evolution. Once the hydrogen fuel in the stellar core is exhausted nuclear burning moves further and further away from the center; the star responds by expanding and thus starts its giant evolution. As discussed by Iben (1967), the rapidly expanding outer convection zones of giants are in contact with the shell burning zones and may again transport nitrogen-enriched material to the surface.