Physical parameters for subdwarf B stars with composite spectra

R. Aznar Cuadrado,  C.S. Jeffery, 
Armagh Observatory, College Hill, Armagh BT61 9DG

New intermediate-resolution spectra have been obtained for a number of 
subdwarf B stars having both single and composite spectra. 
Physical parameters have been determined for the sdB stars and, in 
composite-spectrum systems, their cool companions. For these
binaries, we have developed a method which uses the 
blue-optical spectrum to determine the effective temperatures of
both stars, the surface gravity of the hot stars and the radius
ratio of the system. The surface gravity of the cool star is
measured using the infrared calcium triplet. 
The surface gravities of these cool companions
identify them as main-sequence stars with masses in the range
$0.8-1.2 M_{\odot}$,  confirming a previous energy 
distribution analysis. There is also evidence that the 
composite-spectrum sdBs are more helium-poor than single-spectrum
sdBs.