2 Observations and data reduction

The observations used in this analysis have been collected from the William Herschel (4.2 m) and the Isaac Newton (2.5 m) Telescopes at the Roque de los Muchachos Observatory on La Palma between 1997 and 1999. Instrumental setups for each observing run are given in Table 1.

The stars in the current analysis were all selected because they had been observed with IUE and their flux distributions were well understood (Aznar Cuadrado 2001). The possibility that composite spectrum systems might be chance alignments has also been examined for these stars (ibid.). The sample included a number of stars known to have composite spectra or to have an infrared colour excess, as well as a comparable number considered to be "single-spectrum'' systems. The log of observations is given in Table 2.

   

Table 2: Spectroscopic observations of single and composite sdB stars.

Star	Telesc.	HJD	$\lambda_{\rm cen}$	exp	S/N
		(-2450000)	(Å)	(s)
PG0004+133	WHT	705.704	4400	1200	80
PG0110+262	WHT	704.639	4400	1800	92
	WHT	705.733	8400	1800	74
PG0229+064	WHT	1091.702	4400	200	140
PG0240+046	WHT	1091.721	4400	850	140
PG0342+026	WHT	705.748	4400	900	93
PG0749+658	INT	1264.399	4250	300	70
	INT	1264.538	8400	600	48
PG0839+399	INT	1265.387	4250	600	50
PG1104+243	INT	1265.667	4250	300	52
	INT	1264.596	8400	300	65
PG1233+427	INT	1265.591	4250	150	60
PG1701+359	WHT	704.359	4400	900	150
	WHT	704.359	8400	900	73
PG1718+519	WHT	705.377	4400	1200	80
	WHT	705.378	8400	1200	60
PG2110+127	WHT	704.399	4400	1200	100
	INT	696.543	8400	900	90
PG2135+045	WHT	1090.366	4400	900	70
	INT	696.580	8400	1800	35
PG2148+095	WHT	705.427	4400	1200	170
	WHT	705.426	8400	1200	100
PG2259+134	WHT	704.454	4400	1800	65

All stellar spectra were bias subtracted, flat-fielded, sky subtracted and one-dimensional spectra were extracted using standard IRAF packages. Copper-argon-neon comparison spectra were used to provide a wavelength calibration corresponding to each stellar observation. Spectra were normalized with respect to the local continuum.

The blue spectra are dominated by the hydrogen Balmer series, which are both temperature and gravity sensitive. Other prominent features present in both single and composite-spectrum subdwarfs are the He I$\lambda$4388Å and He I$\lambda$4471Å lines, the magnesium doublet at $\lambda$4481 Å, the silicon triplet at $\lambda\lambda$ 4553, 4568 and 4575 Å, and the C II$\lambda$4267Å doublet, amongst others. The near-infrared spectra of composite sdBs are dominated by the calcium triplet at $\lambda\lambda$ 8498, 8542 and 8662 Å. There are many weaker unidentified features. Several broad hydrogen Paschen lines come from the hot subdwarf. These are effectively invisible because the subdwarf spectrum is swamped by the cool star. However, they have the effect of depressing the apparent continuum in this spectral range.