Physical parameters of the O6.5V+B1V eclipsing binary system LS 1135

The 'All Sky Automated Survey' (ASAS) photometric observations of LS 1135, an O-type single-lined binary (SB1) system with an orbital period of 2.7 d, show that the system is also eclipsing. This prompted us to re-examine the spectra used in the previously published spectroscopic orbit. Our new analysis of the spectra obtained near quadratures, reveals the presence of faint lines of the secondary component. We present for the first time a double-lined radial velocity orbit and values of physical parameters of this binary system. These values were obtained by analysing the ASAS photometry jointly with the radial velocities of both components by performing a numerical model of this binary based on the Wilson-Devinney method. We obtained an orbital inclination i ∼ 68°5. With this value of the inclination, we deduced masses M 1 ∼ 30 ± 1 M ⊙ and M 2 ∼ 9 ± 1 M ⊙ , and radii R 1 ∼ 12 ± 1 R ⊙ and R 2 ∼ 5 ± 1 R ⊙ for primary and secondary components, respectively. Both the components are well inside their respective Roche lobes. Fixing the T eff of the primary to the value corresponding to its spectral type (06.5V), the T eff obtained for the secondary component corresponds approximately to a spectral type of B1V. The mass ratio M 2 /M 1 ∼ 0.3 is among the lowest known values for spectroscopic binaries with O-type components.