1 Introduction

During a systematic search for periodic signals in a sample of $\sim$4000 ROSAT HRI (0.1-2.4 keV) light curves we discovered 321.25s pulsations in the X-ray flux of RXJ0806.3+1527 (Israel et al. 1999, thereafter I99). Based on the large pulsed fraction ($\sim$100%), relatively low 0.5-2.0 keV flux (3.0-5.0$\times$10^-12 erg cm^-2 s^-1), modest distance (edge of the Galaxy is at $\leq$1kpc in the direction of the source) and presence of a faint (B = 20.7) blue object in the Digitized Sky Survey 1 $.\!\!^{\prime\prime}$5 away from the X-ray position, the source was tentatively classified as a cataclysmic variable of the intermediate polar class (I99). A similar classification was also suggested by Beuermann et al. (1999; see also Burwitz & Reinsch 2001) based on the source X-ray colours in the ROSAT all sky survey (RASS).

Figure 1: VLT FORS2 B filter image of the field of RXJ0806.3+1527. The circles represents the X-ray uncertainties inferred from the ROSAT HRI observations. Coordinates units are right ascension and declination.

   

Table 1: Optical observations carried out in 1999-2001 for RXJ0806.3+1527.

Obs.	Telescope & Instrument	Date	Exp.	Range	Comments
#			(s)	(Band/Å)
A	8.2 m VLT-UT2 FORS2	1999/Nov./12	300	BVRIH$_{\alpha}$H $_{\alpha {\rm cont}}$
B	''	1999/Nov./16-19	10500	3600-6000	6 Å res.; grism 600B; slit 1 $^{\prime\prime}$
C	''	1999/Nov./18	600 $\div$ 300	UBVRI
D	''	2000/Feb./05	2700	3600-9000	30 Å res.; grism 150I; slit 1 $^{\prime\prime}$
E	3.6 m ESO EFOSC2	2000/Mar./30	320	UBVRI
F	''	''	1800	B	TRP; exposures of 20s each
G	3.6 m TNG DoLoReS	2001/Jan./01	21600	BVR	TRP; exposures of 20s each
H	8.2 m VLT-UT1 FORS1	2001/Jan./16-23	18000	3600-6000	6 Å res.; grism 600B; slit 1 $^{\prime\prime}$
I	''	2001/Jan./19-24	10800	6000-9000	30 Å res.; grism 150I; slit 1 $^{\prime\prime}$

In this letter we report the results of optical campaigns carried out in 1999-2001, which confirm unambiguously the identification of the optical counterpart of RXJ0806.3+1527 suggested by I99 and provide evidence that RXJ0806.3+1527 is a double white dwarf interacting binary with the shortest known orbital period.