This paper is a part of our ongoing project on multiwavelength optical observations aimed at studying the chromosphere of active binary systems. For this purpose we use the information provided by several optical spectroscopic features that are formed at different heights in the chromosphere (see Montes et al. 1997, Paper I; Montes et al. 1998, Paper II; Montes et al. 2000, Paper III). In addition to study stellar activity, the high resolution spectroscopic observations we use in this project allow us to determine radial velocities and to obtain and improve fundamental stellar parameters.

In this paper we focus our attention on the X-ray/EUV selected chromospherically active binary BK Psc (2RE J003939+103925, BD+09 73, G 1-10, LHS 1118). It is a high proper-motion star with photometry reported by Stephenson (1986), Sandage & Kowal (1986) and Weis (1991) (V= 10.5; U-B = 0.97; B-V = 1.17; V-R = 0.73; R-I = 0.60). Bidelman (1985) gives a K5 spectral type for this star that was confirmed later by Jeffries et al. (1995), but Stephenson (1986) listed it as K4:p. Due to a blue excess in the (U-B) color index the presence of a white dwarf companion has been suggested by Weis (1991) and Cutispoto et al. (1999). Radial velocity variations indicate that it is a binary system (Jeffries et al. 1995; Cutispoto et al. 1999), but no orbital solution has been determined until now. BK Psc was detected as an extreme ultraviolet (EUV) source by the ROSAT Wide Field Camera all-sky survey (Pounds et al. 1993; Pye et al. 1995). The chromospheric activity of this star was detected in the optical identification program of ROSAT EUV Sources by Mason et al. (1995) (EW(H $\alpha$ ) = 1.1 Å and EW(Ca II K) = 2.7 Å) and Jeffries et al. (1995) (strong H $\alpha$ emission above the continuum, EW(H $\alpha$ ) = 1.0 Å). Finally, Cutispoto et al. (1999) found that the best fit for their observed colors (V= 10.43; U-B = 0.92; B-V = 1.16; V-R = 0.73; V-I = 1.49) and HIPPARCOS distance (d=32.8 pc) is obtained by assuming a K5V or K6V primary, a M4V secondary and a possible white dwarf (WD). These authors also confirm the optical variability of this star (photometric period $P_{\rm phot}=2.24\pm0.04$ days), previously reported in the SAAO Annual report (1993) ( $P_{\rm phot}=2.17$ days).

Table 1: Observing log.

2.2 m-FOCES 1999/07 INT-MUSICOS 2000/08 NOT-SOFIN 2000/11 2.2 m-FOCES 2001/09

Day UT Exp S/N Day UT Exp S/N Day UT Exp S/N Day UT Exp S/N

(s) H $\alpha$ (s) H $\alpha$ (s) H $\alpha$ (s) H $\alpha$

26 02:45 2400 87 11 04:19 4000 36 6 01:03 3600 84 24 01:17 1600 32

28 03:36 2000 68 12 04:14 4000 64 6 23:06 3600 79 24 22:45 1600 57

30 03:11 2000 75 13 04:39 4000 62 8 00:28 3600 88

14 04:36 3600 49 8 23:30 3600 96

**Table 1:** Observing log.
2.2 m-FOCES 1999/07		INT-MUSICOS 2000/08		NOT-SOFIN 2000/11		2.2 m-FOCES 2001/09
Day	UT	Exp	S/N		Day	UT	Exp	S/N	Day	UT	Exp	S/N	Day	UT	Exp	S/N
		(s)	H $\alpha$				(s)	H $\alpha$			(s)	H $\alpha$			(s)	H $\alpha$
26	02:45	2400	87		11	04:19	4000	36	6	01:03	3600	84	24	01:17	1600	32
28	03:36	2000	68		12	04:14	4000	64	6	23:06	3600	79	24	22:45	1600	57
30	03:11	2000	75		13	04:39	4000	62	8	00:28	3600	88
					14	04:36	3600	49	8	23:30	3600	96

In this paper we present high resolution echelle spectra of BK Psc, obtained at different epochs, that allow us to measure radial and rotational velocities by using the cross-correlation technique. With these observations we confirm the binary nature (SB1) of this system and determine its orbital solution for the first time. We have obtained an orbital period of 2.17 days, very close to the photometric period of 2.24 days, indicating nearly synchronous rotation. Furthermore, we have applied the spectral subtraction technique to study the chromospheric excess emission in the Ca II H & K, Ca II IRT, H $\alpha$ and other Balmer lines from the primary and secondary components of the system. Preliminary results for this system can be found in Gálvez et al. (2001) and Montes et al. (2001a).

In Sect. 2 we give the details of our observations and data reduction. In Sect. 3 the procedures to obtain the stellar parameters and the orbital determination of the binary system are described in more detail and the results are discussed. The individual behavior of the different chromospheric activity indicators is described in Sect. 4. Finally in Sect. 5 we give the conclusions.

**Table 2:** Stellar parameters of BK Psc.
$T_{\rm sp}$	SB	V	B-V	V-R	$P_{\rm orb}^{1}$	$P_{\rm phot}$	$v\sin{i}^{1}$	$\pi$	$\mu$ $_{\alpha}$ cos $\delta$	$\mu$ $_{\delta}$
					(days)	(days)	(km s^-1)	(mas)	(mas/yr)	(mas/yr)
K5/6:V/M4:V + WD	1	10.43	1.16	0.73	2.1663	2.24	17.1	30.52 $\pm$ 1.79	524.9 $\pm$ 2.1	-198.0 $\pm$ 2.1

¹ values determined in this paper.

1 Introduction