1 Introduction

Subdwarf B (sdB) stars are the most extreme of horizontal branch stars, being predominantly helium stars of approximately half a solar mass overlaid by a hydrogen-rich veneer (Heber 1986). Common in both our own galaxy (Green et al. 1986) and in giant ellipticals (Brown et al. 1997), they present a problem for stellar evolution theory: how does a red giant star dispose of its entire hydrogen-rich envelope prior to core helium ignition?

From an initial supposition that sdB stars were predominantly single, models including enhanced mass-loss rates (e.g. D'Cruz et al. 1996), and white dwarf mergers (Iben 1990; Saio & Jeffery 2000) have been investigated. However a significant fraction of sdB stars are known to have composite spectra (Ferguson et al. 1984; Allard et al. 1994; Jeffery & Pollacco 1998), leading to suggestions of a binary fraction between 50% and 100%. Recent radial velocity studies (Saffer et al. 2001) have identified three distinct groups: 1) single-spectrum sdBs with small or negligible velocity variations, 2) single-spectrum sdBs with large velocity variations and likely periods of hours to days and 3) composite-spectrum sdBs with small velocity variations and relatively long periods. Another recent investigation finds that the second group comprises some $60\pm8\%$ of all sdBs (Maxted et al. 2001). The clear conclusion is that binary evolution plays a significant rôle in the formation of sdB stars.

For the group (2) sdBs, the binary companion is invisible. Radial velocity and, in some cases, light curve studies will yield vital clues about the overall dimensions of these binary systems, and hence about their previous evolution. For group (3), dynamical information is less accessible - although very careful observations over a long time base will be an important tool in this endeavour. Fortunately and by definition, the binary companion in a composite spectrum can be seen.

 

 

Table 1: Instrumental configurations.

Dates	Telesc.	Spec.	Grating	Dichroic	Slit	Detector	R	$\lambda$/Å
1997 Sep. 4	INT	IDS	R1200R	-	1.6''	TEK3	5000	8000-8800
1997 Sep. 12,13	WHT	ISIS	R1200B	5700	1.2''	TEK1	4000	4200-4650
			R600R	5700	1.2''	TEK2	4300	8000-8850
1998 Oct. 3,4	WHT	ISIS	R1200B	5300	1.2''	TEK1	4000	4200-4650
1999 Mar. 26,27	INT	IDS	R1200B	-	1.2''	TEK5	2500	3800-4700
			R1200R	-	1.2''	TEK5	5000	8000-8850

We have already examined the flux distributions for a number of binaries (Aznar Cuadrado & Jeffery 2001: Paper I) and concluded that the companions are main sequence stars. This contradicted previous analyses (Allard et al. 1994; Jeffery & Pollacco 1998) which suggested that the companions were overluminous. Therefore it is important to verify the results spectroscopically. Such an approach carries an additional bonus. The near-infrared triplet lines of ionized calcium are very strong in late-type stars and provide a sensitive diagnostic of surface gravity, providing the effective temperature and metallicity are known (Cohen 1979; Jones et al. 1984; Smith & Drake 1987; J$\o$rgensen et al. 1992). If both surface gravities and the radius ratio can be measured, the mass ratio can be determined directly and provides a very important tool for exploring the previous evolution of this group of sdB stars.

In this paper we introduce the methods used to analyse the spectra of composite-spectrum sdB stars, and present results for an initial sample. The methods are tested by providing independent analyses for a number of single-spectrum sdB stars.