We report 6 yr monitoring of a distant bright quasar CTS C30.10 (z = 0.90052) with the Southern African Large Telescope (SALT). We measured the rest-frame time-lag of 562±2 days between the continuum variations and the response of the Mg II emission line, using the Javelin approach. More conservative approach, based on five different methods, imply the time delay of 564 +109 −71 days. This time delay, combined with other available measurements of Mg II line delay, mostly for lower redshift sources, shows that the Mg II line reverberation implies a radius-luminosity relation very similar to the one based on a more frequently studied Hβ line.

We present a detailed calculation of the evolution of low-mass (,0.25 M ( ) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M ( . As a result of mixing between the core and outer envelope, the surface hydrogen content (X surf ) is 0.5±0.35, depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.The duration of the flashes (independent of chemical composition) is between a few Â10 6 and a few Â10 7 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M ( (depending on the heavy element value). Spruit 1987;Muslimov & Sarna 1995) and, as a consequence, the system loses orbital angular momentum. For a magnetic stellar wind we used the formula for the orbital angular momentum loss _ J J MSW 23 Â 10 27 M 2 R 2 2

We present evolutionary sequences for low‐mass close binary systems in which a low‐mass (1.0–1.5 M⊙) secondary star transfers mass to a neutron star. Roche lobe filling occurs when the secondary is a turn‐off main‐sequence star (having a small helium core). We assume loss of angular momentum owing to gravitational wave radiation and magnetic braking. We have found that the loss (and the mechanism of loss) of mass and angular momentum from the system is the main factor determining the value of the bifurcation period (Pbif). The bifurcation period separates the formation of the converging systems from the diverging systems. Variations in the initial chemical composition, and in the initial mass of the secondary, lead only to minor changes in Pbif. We have also investigated how changes in the chemical composition influence the initial orbital period (Pi) versus final orbital period (Pf) relation. The initial chemical composition has a more significant effect on this relation for shorter Pi than for longer Pi. We have found systematic differences for the Pf versus white dwarf mass relation for various chemical compositions. For converging systems, we have found that there is a boundary orbital period (Pb) such that if Pi < Pb, a system will evolve through the period gap (there are no low‐mass X‐ray binaries with orbital periods between one and three hours) with a Roche lobe overflowing secondary, but the accretion on to the neutron star is forbidden owing to the ‘propeller effect’. The systems will end their evolution as ultra‐short period and very bright, low‐mass X‐ray binaries. If Pb < Pi < Pbif, then short orbital period millisecond binary pulsar systems will be formed.

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A B S T R A C TUsing a Paczyn Âski-type stellar evolution code, we have modelled the evolution of secondary stars through the common-envelope and semidetached phases of close binary evolution. Our calculations are aimed at determining the effects of the accretion of red giant envelope material during the common-envelope phase, and the accretion of novae ejecta during the semidetached phase, on the surface composition of the secondary. For the second case, we make use of the results of the calculations of Prialnik & Kovetz, Kovetz & Prialnik and Prialnik, which concern the effects of accretion on to carbon±oxygen white dwarfs with varying white dwarf mass, white dwarf central temperature and mass transfer rate (the de®ning characteristics of nova outburst behaviour).In this paper we present the results of our calculations of a large grid of models designed to test the effects of these two polluting processes, both individually and together. We ®nd that both processes may signi®cantly affect the surface composition of the secondary, and that in the case of the accretion of novae ejecta, thermohaline mixing is extremely important and cannot be neglected. Our results indicate four phases in the evolution of the surface composition of the secondary during semidetached evolution: (i) the material accreted during the common-envelope phase and that accreted from the novae ejecta are mixed by convection and thermohaline mixing and dominate the surface composition of the secondary; (ii) the material accreted during common-envelope evolution is lost through mass transfer and the surface composition of the secondary is dominated by material accreted from the novae ejecta, which is mixed with the underlying layers of the secondary by convection and thermohaline mixing; (iii) the surface convection zone of the secondary penetrates to deeper layers so that these deeper layers dominate the surface composition of the secondary and thermohaline mixing no longer operates; and (iv) for evolved Population I systems, when the secondary has lost most of its mass, its surface composition is again dominated by material accreted from novae ejecta.We make predictions concerning the abundances of carbon, nitrogen and oxygen and the values of the isotopic ratios 12 C/ 13 C, 14 N/ 15 N and 16 O/ 17 O on the surface of the secondary throughout its evolution. With observationally determined abundances and isotopic ratios, we can in principle determine whether or not these two polluting processes operate, and can further differentiate between them.

We examine the possibility of probing dynamo action in mass‐losing stars, components of Algol‐type binaries. Our analysis is based on the calculation of non‐conservative evolution of these systems. We model the systems U Sge and β Per where the more massive companion fills its Roche lobe at the main sequence (case AB) and where it has a small helium core (early case B) respectively. We show that to maintain evolution of these systems at the late stages which are presumably driven by stellar ‘magnetic braking’, an efficient mechanism for producing large‐scale surface magnetic fields in the donor star is needed. We discuss the relevance of dynamo operation in the donor star to the accelerated mass transfer during the late stages of evolution of Algol‐type binaries. We suggest that the observed X‐ray activity in Algol‐type systems may be a good indicator of their evolutionary status and internal structure of the mass‐losing stellar components.

Using six years of spectroscopic monitoring of the luminous quasar HE 0435-4312 (z = 1.2231) with the Southern African Large Telescope, in combination with photometric data (CATALINA, OGLE, SALTICAM, and BMT), we determined a rest-frame time delay of days between the Mg ii broad-line emission and the ionizing continuum using seven different time-delay inference methods. Time-delay artifact peaks and aliases were mitigated using the bootstrap method and prior weighting probability function, as well as by analyzing unevenly sampled mock light curves. The Mg ii emission is considerably variable with a fractional variability of ∼5.4%, which is comparable to the continuum variability (∼4.8%). Because of its high luminosity (L 3000 = 1046.4 erg s−1), the source is beneficial for a further reduction of the scatter along the Mg ii-based radius–luminosity relation and its extended versions, especially when the highly accreting subsample that has an rms scatter of ∼0.2 dex is considered. This opens up the possibility of using the high-accretor Mg ii-based radius–luminosity relation for constraining cosmological parameters. With the current sample of 27 reverberation-mapped sources, the best-fit cosmological parameters (Ωm, ΩΛ) = (0.19; 0.62) are consistent with the standard cosmological model within the 1σ confidence level.

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