6 Y Sex

The eclipsing binary Y Sex (also HD 87079, BD  $+01^{\circ}$ 2394, HIP 49217, PPM 156443, AN 358.1934, FL 1156, P 3375; $\alpha_{2000} = 10^{\rm h}02^{\rm m}48.0^{\rm s}$, $\delta_{2000} =$ $+01^{\circ}05\hbox{$^\prime$ }40.3\hbox{$^{\prime\prime}$ }$, $V_{\max}=9.97$ mag; Sp. F8) is a relatively bright and well-known A-type contact binary with an extremely low mass ratio. It was discovered to be a variable star by Hoffmeister (1934). Later, the system was observed photographically by Prikhodko (1947) and Gaposchkin (1953). The first photoelectric observations of Y Sex were presented by Tanabe & Nakamura (1957) and Hill (1979) who derived a relatively low value of the mass ratio of the components q = 0.175. Radial velocities were obtained by McLean & Hilditch (1983), resulting in $q = 0.18 \pm 0.03$, with good agreement with the photometrically determined value. Herczeg (1993), in his period study, derived these linear light elements:

$${\rm Pri.~Min.} = {\rm HJD~24~34445.9912} + 0\hbox{$.\!\!^{\rm d}$ }41981391 \cdot E.$$

Recently, another period study of Y Sex was published by Qian & Liu (2000). They presented the quadratic light elements

$$\begin{eqnarray*}{\rm Pri.~Min.} &=& {\rm HJD~24~34445.9786}\\ && + 0\hbox{$.\!... ...$ }41981527 \cdot E - 3\hbox{$.\!\!^{\rm d}$ }14 \ 10^{-11} E ^2 \end{eqnarray*}$$

and interpreted the secular decrease of the period as mass transfer from the more to the less massive component or mass and angular momentum loss from the system.

 

 

Table 3: Eclipsing binaries with long-term continuos increase of the period

System	Spectral	Period	${\rm d}P/{\rm d}E$	${\rm d}M/{\rm d}t$	Reference
	type	[days]	[10-10 days/cycle]	[ $10^{-7}~M\odot$/year]
AP Aur*	A2	0.56937	18.13	15.5	Agerer & Splittgerber (1993)
BX Dra*	A3	0.57903	11.12	**	Agerer & Dahm (1995)
XY Boo	F5V	0.37055	6.20	1.34	Molík & Wolf (1998)
UZ Leo	A7	0.61804	6.07	1.30	Hegedüs & Jäger (1992)
V839 Oph	F8V	0.40900	3.46	3.31	Wolf et al. (1996)
AH Vir	K0V	0.40752	2.66	0.76	Demircan et al. (1991)
GO Cyg *	B9+A0	0.71776	2.26	4.01	Sezer et al. (1985), Rovithis (1997)
V401 Cyg	F0	0.58272	1.48	0.20	this paper
44 i Boo	G2V+G2V	0.26782	1.24	1.15	Gherega et al. (1994)
DK Cyg	A8V	0.47069	1.15	0.29	this paper
CT Eri	F0	0.63420	1.02	0.38	Lipari & Sistero (1987)

Notes: * EB type eclipsing binary, ** data not available.

Surprisingly, the spectral type F8 given in Hill (1979) and mentioned also in Herczeg (1993) doesn't agree with the mass determination by Kaluzny (1985), $M_{\rm tot} = 0.81~M_{\odot}$. Apart from the eclipses included in Table 1 by Herczeg (1993) we have adopted, for a new determination of the period change of Y Sex, the numerous minima given by Agerer (1986, 1988, 1989, 1991, 1992, 1993), Agerer & Hübscher (1998a, 1998b), Agerer et al. (1999) and Diethelm (1995). Using this data base, which includes 42 more reliable timings, we propose another explanation of the current O-C diagram in Fig. 7.

Figure 7: O-C diagram for Y Sex. The curve corresponds to a third body orbit

The sinusoidal variation of the period is remarkable and could be caused by a light-time effect. A preliminary analysis of the third body orbit gives the following parameters:

P₃ (period) = $21\, 050 \pm 50$ days, i.e. 57.6 years;
T₀ (time of periastron) = JD $24 50150 \pm 50$;
A (semiamplitude) = $0.0181 \pm 0.0002$ day;
e₃ (eccentricity) = $0.52 \pm 0.06$;
$\omega_3$ (length of periastron) = $320.0^{\circ} \pm 1.2 ^{\circ}$.

These values were obtained together with the new mean linear ephemeris

${\rm Pri.~Min.} = {\rm HJD~24~34445.9695} + 0\hbox{$.\!\!^{\rm d}$ }41981485 \cdot E,$

$\pm 0.0002$ $\pm 0.00000001$

by the least squares method. Assuming a coplanar orbit ( $i_3 = 90^{\circ}$) and a total mass of the eclipsing pair according to the spectral type F8, $M_1 + M_2 = 1.2~M_{\odot}$(Harmanec 1988), we can obtain a lower limit for the mass of the third component $M_{\rm 3, min}$. The value of the mass function is $f(M)= 0.0120~M_{\odot}$, from which the minimum mass of the third body follows as $0.3~M_{\odot}$. A possible third component of spectral type M4-M5 with the bolometric magnitude about +9.6 mag could be practically invisible in this system with an F8 primary ( $M_{\rm bol} \simeq +3.9$ mag). Therefore, new high-accuracy timings of this eclipsing binary are necessary in order to confirm the light-time effect in this system.