We consider the spin angular momentum evolution of the accreting components of Algol‐type binary stars. In wider Algols the accretion is through a disc so that the accreted material can transfer enough angular momentum to the gainer that material at its equator should be spinning at breakup. We demonstrate that even a small amount of mass transfer, much less than required to produce today's mass ratios, transfers enough angular momentum to spin the gainer up to this critical rotation velocity. However the accretors in these systems have spins typically between 10 and 40 per cent of the critical rate. So some mechanism for angular momentum loss from the gainers is required. Unlike solar‐type chromospherically active stars, with enhanced magnetic activity which leads to angular momentum and mass loss, the gainers in classical Algols have radiative envelopes. We further find that normal radiative tides are far too weak to account for the necessary angular momentum loss. Thus enhanced mass loss in a stellar wind seems to be required to spin‐down the gainers in classical Algol systems. We consider generation of magnetic fields in the radiative atmospheres in a differentially rotating star and the possibility of angular momentum loss driven by strong stellar winds in the intermediate‐mass stars, such as the primaries of the Algols. Differential rotation, induced by the accretion itself, may produce such winds which carry away enough angular momentum to reduce their rotational velocities to the today's observed values. We apply this model to two systems with initial periods of 5 d, one with initial masses 5 and and the other with 3.2 and . Our calculations show that, if the mass outflow rate in the stellar wind is about 10 per cent of the accretion rate and the dipole magnetic field is stronger than about 1 kG, the spin rate of the gainer is reduced to below breakup velocity even in the fast phase of mass transfer. Larger mass loss is needed for smaller magnetic fields. The slow rotation of the gainers in the classical Algol systems is explained by a balance between the spin‐up by mass accretion and spin‐down by a stellar wind linked to a magnetic field.

We have compiled the well-determined absolute parameters of Algol-type binaries. The lists contain the parameters of 74 detached and 61 semidetached close binaries. The double-lined eclipsing binaries provide not only the most accurate determinations of stellar mass, radius and temperatures but also distance-independent luminosity for each of their individual components. The distributions of the primary and secondary masses of detached binaries (DBs) are similar, whilst the secondary masses of the semidetached binaries (SDBs) are mostly smaller than 2 M with a peak in the M 2 -bin (0.21-1.0). The components of the DBs are almost all located in the main-sequence band. On the contrary, the secondary components of the SDBs have larger radii and luminosity with respect to the same mass and the same effective temperature of main-sequence counterparts. They occupy a region of the Hertzsprung-Russell diagram between terminal-age main sequence and giants. Moreover, the total angular momenta and specific angular momenta are larger for the SDBs of orbital periods with P > 5 d than those of the shorter period ones. The specific angular momenta of SDBs with periods longer than 5 d are 65 per cent greater than that of the short period group with the same mass. The DBs and the SDBs with orbital periods longer and shorter than 5 d are separated into three groups in the J/M 5/3 − q diagram. The SDBs with mass ratios greater than 0.3 and P > 5 d have almost the same angular momentum to those of DBs. However, the SDBs with short periods have the smallest angular momentum even though they have the same mass ratios. This result reveals that angular momentum loss (AML) considerably affects the evolution of close binary systems. Recently, Chen, Li & Qian suggested that, in addition to magnetic braking, a circumbinary disc may play an important role in AML from Algol-type binaries. Their calculations indicated that the evolution of Algol-type binaries can be significantly affected by the circumbinary disc. Our results show that the evolution of close binaries begins as a DB and losing angular momentum, first via stellar wind and then magnetic braking plus circumbinary disc the period is shortened and orbit shrinks. Thereafter, the evolution of the system is accelerated and mass transfer rates are enhanced which result in a smaller mass ratios.

The chemical composition of stellar photospheres in mass-transferring binary systems is a precious diagnostic of the nucleosynthesis processes that occur deep within stars, and preserves information on the components' history. The binary system u Her belongs to a group of hot Algols with both components being B-stars. We have isolated the individual spectra of the two components by the technique of spectral disentangling of a new series of 43 high-resolutioń echelle spectra. Augmenting these with an analysis of the Hipparcos photometry of the system yields revised stellar quantities for the components of u Her. For the primary component (the mass-gaining star) we find M A = 7.88 ± 0.26 M ⊙ , R A = 4.93 ± 0.15 R ⊙ and T eff,A = 21 600 ± 220 K. For the secondary (the mass-losing star) we find M B = 2.79 ± 0.12 M ⊙ , R B = 4.26 ± 0.06 R ⊙ and T eff,B = 12 600 ± 550 K. A non-LTE analysis of the primary star's atmosphere reveals deviations in the abundances of nitrogen and carbon from the standard cosmic abundance pattern in accord with theoretical expectations for CNO nucleosynthesis processing. From a grid of calculated evolutionary models the best match to the observed properties of the stars in u Her enabled tracing the initial properties and history of this binary system. We confirm that it has evolved according to case A mass transfer. A detailed abundance analysis of the primary star gives C/N = 0.9, which supports the evolutionary calculations and indicates strong mixing in the early evolution of the secondary component, which was originally the more massive of the two. The composition of the secondary component would be a further important constraint on the initial properties of u Her system, but requires spectra of a higher signal to noise ratio.

Algol-type binary systems are the product of rapid mass transfer between the initially more massive component to its companion. It is still unknown whether the process is conservative, or whether substantial mass is lost from the system. The history of a system prior to mass exchange is imprinted in the photospheric chemical composition, in particular in the carbon-to-nitrogen (C/N) ratio. We use this to trace the efficiency of mass-transfer processes in the components of a classical Algol-type system, δ Librae. The present analysis is based on new spectroscopic data (groundbased high-resolutionéchelle spectra) and extracted archival photometric observations (space-based measurements from the STEREO satellites). In the orbital solution, non-Keplerian effects on the radial-velocity variations were taken into account. This reduces the primary's mass by 1.1 M ⊙ (∼23%) significantly in comparison to previous studies, and removes a long-standing discrepancy between the radius and effective temperature. A spectral disentangling technique is applied to theéchelle observations and the spectra of the individual components are separated. Atmospheric and abundance analyses are performed for the mass-gaining component and we found C/N = 1.55 ± 0.40 for this star. An extensive set of evolutionary models (3.5 × 10 6 ) for both components are calculated from which the best-fitting model is derived. It is found that β, the parameter which quantifies the efficiency of mass-loss from a binary system, is close to zero. This means that the mass-transfer in δ Lib is mostly conservative with little mass loss from the system.