1 Introduction

Thejll et al.$\,$(1995; hereafter Paper I) discussed the possible origin for excess infrared fluxes detected from the direction of a hot subdwarf star (sd). They presented a set of JHK measurements for 27 hot objects: 23 hot sds, GD274 -which the authors reclassify as sd+K3-K8, and 3 white dwarfs (WDs). An analysis was also presented for the, then 24, hot subdwarfs. After considering a wind expelled from the hot atmosphere as the source of the excess IR radiation, the authors concluded that the most likely explanation is the presence of a cool stellar companion giving rise to the observed excess. With the aid of comprehensive literature and database searches, several well known cases were corroborated, suspected cases confirmed and new discoveries uncovered.

The point in studying companions to hot subdwarfs is that we can learn things about the hot subdwarfs that are not easy to observe directly due to the problems of modeling hot atmospheres and the relatively large distances to these stars that preclude some astrometric analysis for the majority of these stars. Parallaxes are hard to measure but some are expected from the HIPPARCOS data for a small number of close stars. Proper motion analysis has been performed by Thejll et al.$\,$(1997) Spectral analyses of hot subdwarf B stars (e.g. Saffer et al.$\,$1994) and sdO stars (Thejll et al.$\,$1994a) exist. Our work presents an investigation on the properties of the companion stars, and the hot subdwarf properties that can be inferred from these.

Following a similar approach to that in Paper I, the results presented here relate to 72 more stars (63 hot sds; 1 Horizontal Branch B (HBB) star - PG 0342+026; 7 WDs; and the non-sd object PHL 382 - Kilkenny, private communication) observed with the same telescope and instrumentation. In the present paper, among other means of analysis, they are analysed following the methods described in Paper I but the UV, optical and IR fluxes are interpreted with the aid of Kurucz spectral models - in Paper I black body (BB) functions were used. Objects in Paper I are therefore reanalyzed and the corresponding results included here. Out of the 99 (=27 + 72) stars observed with the CST in 1994, the total number of hot subdwarf/HBB stars studied, i.e., 88 (=23 + GD 274, from February, + 63 + PG 0342+026, from June and October), basically corresponds to the total possible number of this kind of objects observable, as extracted from Kilkenny et al.$\,$(1988), given the limiting magnitude of the instrument employed ($\sim$13.5 mag in K). This total sample is, to our knowledge, the most complete JHK hot subdwarf catalogue available to date. Most of these targets are observed at IR wavelengths for the first time. Tables 1 and 2 present the 72 objects added in the present work.

  

Table 1: List of observed objects. Most common names, spectral type, coordinates (Epoch 1950.0 or stated otherwise), temperatures and gravities available in the literature, or estimated in this paper from published photometry and UV fluxes, together with source references for program stars, are given. Notes: $\star$=2000.0 coordinates; 1=This work; pI=Thejll et al.$\,$(1995); A94=Allard et al.$\,$(1994); BH95=Bauer & Husfeld (1995); Bo94=Bowyer et al.$\,$(1994); D90=Dreizler et al.$\,$(1990); D93=Dreizler (1993); G80=Giddings (1980); GS74=Greenstein & Sargent (1974); H86=Heber (1986); HB95=Hurwitz & Bowyer (1995); HL86=Heber & Langhans (1986); Je92=Jeffery et al. (1992); Ji94=Jiménez et al.$\,$(1994); K=Kilkenny, private communication; Ma95=Marilli et al.$\,$(1995); Mo90=Moehler et al.$\,$(1990); N93a=Napiwotzki (1993a); N93b=Napiwotzki (1993b); P70=Peterson (1970); S94=Saffer et al.$\,$(1994); T93=Theissen et al.$\,$(1993); T95=Theissen et al.$\,$(1995); Th96=Thejll, Husfeld & Saffer (unpublished work); To70=Tomley (1970); V91=Viton et al. (1991); Ve93=Vennes et al. (1993); Ve94=Vennes et al. (1994); Wo96=Wolff et al. (1996). Temperatures in parenthesis indicate that there is not enough data to asses the temperature but that an estimate has been made on the basis of literature spectral classes - thus any un-analyzable sdO is set to 40000 K while all un-analyzable sdB or sd are set to 20000 K. HD 113001 and GD 274 were observed in February (1994)

 

Table 2: List of observed objects (continuation). $\dagger$=S94 reports 29700 K and log(g)=4.6 but that would make the contribution from the companion star so dominant in the optical range that the spectral analysis by S94 is meaningless

Our paper consists of a part presenting the JHK observations and two analysis parts. The reader is referred to Paper I for details of these methods. The JHK observations and the reduction procedure are discussed in Sect. 2. Compilation of optical and UV data, and treatment of the latter, is described in Sect. 3. The detection and extraction of excess fluxes using estimates of the $T_{\rm eff}$ of the hot subdwarf is discussed in Sect. 4. We then proceed in Sect. 5 to analyze the excesses and calculate the gravities of the hot subdwarfs assuming that stellar companions are zero-age main sequence (ZAMS) stars. Section 6 presents a summary and discussions.