1 Introduction

Soft X-ray surveys of star forming regions (SFRs), for example of Chamaeleon, Lupus, Taurus and Orion, have revealed groups of low-mass (M_* $\leq 3 \:$$M_\odot$) active stars apparently associated with those regions (e.g. Alcalá et al. 1995, 1996; Wichmann et al. 1996; Krautter et al. 1997). These objects have late spectral types (typically G0 or later) and are pre-main sequence (PMS) or young (age $\leq$ 10⁸ yr) main sequence (MS) stars. They are now known as weak-line T Tauri stars (WTTS). WTTS are apparently associated with SFRs and, assuming that their distances to the observer are those of their associated regions, one infers their youth from their positions in the H-R diagram. Besides these morphological and photometric properties, the WTTS also present key features in their spectra that reveal us their young (age < 10⁸) nature: they have chromospheric activity (H$\alpha$ and CaII H+K lines in emission, W(H$\alpha$) $\leq$ -10 Å), the photospheric line LiI $\lambda \; 6708$ Å is conspicuously present in their spectra) and the stars rotate fast ( $v_{\rm rot} \approx 30$ km s^-1 or more). A necessary condition to classify these objects as PMS stars is the presence of the Li I feature in excess in the W(Li) versus $\log \: T_{\rm eff}$ plane when compared with the upper envelope of the Pleiades stars (e.g. Fig. 12 of Alcalá et al. 1998, hereafter A98). In this case, Lithium has not had enough time to be destroyed in the deeper layers of the convection zone (cf. Bodenheimer 1965). On the other hand, the WTTS are mildly masked, if at all, by circumstellar material: they lack (strong) spectral line and continuum (ultraviolet or infrared) emissions that characterize the classical T Tauri stars (CTTS), which are believed to be younger (age $\leq 10^7$ yr). WTTS  are considered the descendants of CTTS. However, the criteria for establishing the membership of the WTTS to an ongoing star forming region should be taken with caution because of their large spatial distribution ( $> 100 \; \Box$⁰) and because their distance to the observer is, in most cases, unknown.

Although the EINSTEIN satellite shed new light on the evolution of these PMS objects, its observations were spatially biased towards the denser parts of the SFRs, where CTTS predominate and follow a clumpy pattern. Hence, little or no inference could be drawn about the history of star formation at a cloud scale. The spatially unbiased ROSAT All-Sky Survey (RASS) remedies this situation: it yields a spatially complete but flux-limited sample of X-ray sources around a given cloud at about EINSTEIN's sensitivity. Contrary to the spatial distribution of the CTTS, WTTS were found to be uniformly distributed over the whole observed area and outnumber the preceding CTTS by, at least, a factor of 3. The location of the WTTS in the evolutionary sequence has not yet been well established (e.g. Montmerle et al. 1993; Chavarría-K et al. 1995), but in general, stellar activity is considered a quantitative landmark of youth for this type of objects. Since WTTS are less active in the uv, optical and infrared spectral regions than CTTS, the former are considered older than the latter. Consequently, WTTS had more time to disperse from their birth sites, explaining, at least partially, their broader and homogeneous spatial distribution. Despite the intrinsic X-ray brightness of the WTTS, we are luminosity-limited in their detection. At present, we are constrained to only nearby SFRs (d < 500 pc). Many follow-up studies (optical, mostly spectroscopic) of the EINSTEIN and ROSAT surveys of nearby SFRs such as Chameleon, Taurus-Auriga, Lupus, Orion  and Scorpius-Centaurus have recently appeared in the literature (e.g. Alcalá et al. 1995; Wichmann et al. 1996, 1997; Li & Hu 1998; Magazzù et al. 1997; Krautter et al. 1997; Alcalá et al. 1996, 1998; Walter et al. 1988, 1994; Sciortino et al. 1998 and references therein). Unfortunately, photometric data of many WTTS and WTTS candidates associated with the regions of interest are lacking or the stars were observed in an unsuitable photometric system (e.g. the photographically-based GSC or satellite-born V magnitude estimates, particularly for red stars): the exceptions are the monitoring for light-variability of 58 WTTS  (and WTTS candidates) in Taurus by Bouvier et al. (1997) in Johnson's B and V passbands, the uvby-$\beta$ photometry of WTTS and WTTS candidates in Orion by Alcalá et al. (1996, hereafter A96) and the UBVRIJHKL photometry of WTTS in Scorpius OB2-2 by Walter et al. (1994, hereafter W94).

In this paper we report and discuss photometric data of X-ray flux selected and confirmed WTTS and WTTS candidates in the Taurus-Auriga SFR (uvby-$\beta$and JHK, 58 and 20 stars, respectively) and in Scorpius OB2-2 SFR (uvby-$\beta$, 18 stars). We also extend the photometric sample of WTTS in Orion reported by A96 (uvby-$\beta$, 40 stars). The majority of stars are observed for the first time in this photometric system. The data cover about 74% of the objects on the list by Wichmann et al. (1996, hereafter Wi96), about 36% on the list by A96, and about 46% from the list of WTTS and WTTS candidates in the Upper Scorpius Association by Wa94, as well as six objects near the runaway O star $\zeta$ Oph (Oph1 through Oph6, Walter 1986, see also Terranegra et al. 1994).

In spite of previous spectroscopic, photometric and proper motion studies, the membership of WTTS to clusters with ongoing star formation is still an open matter. It is the purpose of this work to cover some of these deficiencies for future analysis and observations.