half the mass of the Sun. The orbit of this star causes perturbation effects on the orbit of the AB pair, resulting in periodic changes in their orbital eccentricity and other orbital elements. Physical characteristics The spectrum of Lambda Tauri A shows an under abundance of carbon relative to the norm for this category of star. A possible explanation for this is a loss of mass by the star some time in the past. An inner region of the star became depleted by the conversion of carbon into nitrogen during the nuclear fusion process, and this region was later exposed when the outer envelope of the star was lost. Alternatively, the star may have undergone a period of convective mixing, bringing the carbon-depleted material to the surface. However, the cause of such a fully convective behavior in a main sequence star of this mass is unclear. The secondary companion has a stellar classification of A4 IV, suggesting that it is a subgiant star that has nearly exhausted the supply of hydrogen at its core and is in the process of evolving into a giant star. It has nearly 1.9 times the mass of the Sun, 5.3 times the Sun's radius, and is radiating 128 times the Sun's luminosity at an effective temperature of . As with the primary, this star is spinning rapidly with a projected rotational velocity of . The side of the secondary facing the more massive star is being heated by an additional , which produces a rotational effect that causes the strength of the secondary's spectral lines to vary over the course of its orbit. A conundrum with this system is the large radius of the secondary star. In stellar evolutionary terms, the more massive primary should be the first to reach the subgiant stage. Hence the enlarged radius of the secondary must be caused by a means other than the star's age. This suggests that the pair Lambda Tauri AB form a semidetached binary with the secondary filling its Roche lobe, giving it a distorted shape. References Algol variables Tauri, Lambda B-type main-sequence stars Taurus (constellation) Triple star systems A-type subgiants Tauri, 035 018724 1239 025204 Durchmusterung objects