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The strange story of BI Lyncis:
an extremely helium-rich \subdwarf" with a cool companion
C.S. Jeery and R. Aznar Cuadrado
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
csj@star.arm.ac.uk, rea@star.arm.ac.uk
Abstract
BI Lyn = PG 0900+400 has been erroneously clas-
sied as an evolved binary system containing either
a hot subdwarf or white dwarf and a thick accretion
disk. New intermediate dispersion spectra are pre-
sented which demonstrate that the hot component is
a luminous low-mass helium star and the cool com-
panion is a rapidly rotating G-type giant. Techniques
of spectrum synthesis have been used to establish
the dimensions of both components. Although the
orbital period of the system remains unknown, other
phenomena are entirely consistent with these observed
dimensions. The established  0:34d periodic pho-
tometric variation could be explained by pulsations
in the B-type primary and, by analogy with other
H-decient binaries, it is suggested that the variable
absorption in H may be due to a gas stream owing
through the inner Lagrangian point.
BI Lyn: the story so far
 Identied in the PG survey as an sdB star
with an infrared excess (Ferguson et al. 1984).
 Photometric monitorring identied a 0.34d
period. (Lipunova & Shugarov 1990, 1991).
 H has a P Cygni prole with variable ab-
sorption (Wade & Potter 1995).
 Suspected velocity variable (Orosz et al.
1997).
 sdB+K3V spectral decomposition (Ferguson
et al. 1984, Orosz et al. 1997, Liu & Hu 2000)
 Reports of strong He i lines (Orosz et al. 1997,
Liu & Hu 2000)
 SIMBAD gives \V* BI Lyn { Nova-like Star"
We present here:
 A reanalysis of the UV and optical ux distribu-
tion to measure the eective temperatures and angular
diameters of both stars (Fig. 1). For this we used the
automatic ux tting package ffit and a combination
of hydrogen-decient model atmospheres for hot stars
and Kurucz models atmospheres for cool stars.
Figure 1. Ultraviolet and visual spectrophotometry
of BI Lyn (histogram) together with the best fitting
theoretical flux distribution (polyline and horizontal
bars). The latter represents the sum of two model
atmospheres (dashed lines) with T eff:1 = 28 600 K,
 1 = 0:55  10 11 rad, T eff:2 = 5 840 K and  2 =
4:09  10 11 rad. Interstellar reddening is negligible.
 New INT and WHT intermediate resolution spec-
tra of BI Lyn in the blue and around H and the in-
frared calcium triplet (Figs. 2 { 4). The new spectra
demonstrate the relative strnegth of the neutral helium
lines relative to the hydrogen Balmer lines.
 A simultaneous spectral analysis of both stars us-
ing the automatic tting package sfit and a combi-
nation of synthetic spectra for hydrogen-decient hot
stars and normal composition cool stars.
Figure 2. Normalized blue spectrum of BI Lyn (bot­
tom: d) together with a best fit composite model spec­
trum (c) formed by adding models with a) T eff:2 =
5 840 K, log g 2 = 3:2, [Fe=H] 2 = 0:00 (top) and b)
T eff:1 = 28 600 K, log g 1 = 3:64, n He1 = 0:95 as­
suming that the relative radii R 2 =R 1 = 4:9. The model
spectra have been velocity shifted and degraded to
match the observed spectral resolution (1 љ A).
 The analysis of the hot star based on the WHT
blue spectrum (Fig. 2) yields a hydrogen abundance
 1% by number, T eff  28 000 K and log g  3:6,
making the primary a helium giant.
Figure 3. As Fig. 2 in the region of the infared calcium
triplet.
 Assuming a solar composition for the cool star, its
surface gravity is estimated from the CaT lines (Fig.
3) to be log g  3:2 and hence also a giant.
 The two methods (ffit and sfit) give a radius ratio
for the two stars R 2 =R 1 between 7.4 and 4.9.
 Simply estimating M 1 = 0:5 M enables all re-
maining system dimensions to be evaluated.
Figure 4. As Fig. 2 in the region of H.
Conclusions
 Contrary to previous assumptions, BI Lyn
does not contain a hot subdwarf or white
dwarf, nor is it a nova-like variable.
 The hot star is a luminous hot hydrogen-
decient star with a probable mass around
0.5 M .
 The cool star is a cool giant with a mass
> 1 M .
 The hydrogen-deciency of the hot star is
probably the result of a comon-envelope phase
during which the outer envelope was entirely
removed or transferred to the cool companion.
 The hot star luminosity suggests that it lies
on a post-AGB evolution track.
 Previously established 0.34d light variations
are likely to be due to pulsations in the he-
lium star.
 By analogy with hydrogen-decient binary 
Sgr, previously established H P-Cygni pro-
le variations may be due to the orbital mo-
tion of a supersonic jet between the compo-
nents.
 Further observations are required to deter-
mine the orbital period and mass ratio, to ver-
ify the pulsation hypothesis and to correlate
the H behaviour with orbital phase.
References
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