Appendix B: Notes on individual sources

RXJ 2331.4-4209: This object is a visual binary with a separation of 0$.\!\!^{\prime\prime}$4, and a difference in magnitude of $\Delta m = 0.8$. The colour indices are consistent with two components of very near spectral types, in agreement with the small $\Delta m$.

RXJ 0053.0-3021: HD 5133. The (B-V) index yields a distance of 12 pc for this object, which is in good accordance with the distance of 13 pc given by the trigonometric parallax ($\pi$ = 0$.\!\!^{\prime\prime}$075, Hipparcos Input Catalogue).

RXJ 0121.5-4058: This is also a visual binary, with a separation of 6$.\!\!^{\prime\prime}$1, and a $\Delta m$ of 5.5. Only one component is therefore detectable in the photometry, the other being too faint.

RXJ 0440.3-5856: It has a very soft X-ray spectrum, and a rather low X-ray luminosity. This is an X-ray quiet object, detected only due to its nearness to the Sun ($\sim$19 pc, or 17 pc according to the measured trigonometric parallax ($\pi$ = 0$.\!\!^{\prime\prime}$059, Hipparcos Input Catalogue)).

RXJ 0505.6-5728: V=4.7. This star was not observed, as it is too bright for the photomultiplier tubes. The colours and spectral classification were taken from the literature (Bessel 1990). The distance determined using those colours agrees very well with the distance derived from the trigonometric parallax (13 pc, $\pi$ = 0$.\!\!^{\prime\prime}$078).

RXJ 0527.6-6024: this star has a measured trigonometric parallax that gives a distance of 25 pc ($\pi$ = 0$.\!\!^{\prime\prime}$040 $\pm$ 0$.\!\!^{\prime\prime}$013, Hipparcos Input Catalogue). The value found using the colour index (B-V) is quite smaller (19 pc), but still within the error box of the distance given by the trigonometric parallax.

RXJ 0545.3-5543: this is another visual binary with a separation of 7$.\!\!^{\prime\prime}$5 and a difference in visual magnitude of $\Delta m$ = 1.8. The photometric colours measured agree well with a binary.

RXJ 0549.7-5950: the photometric colours correspond to a F7/8V star, but the object is a visual binary with a separation of 2$.\!\!^{\prime\prime}$5 and a difference in visual magnitude of $\Delta m$ = 0.4. The spectroscopic observation available shows the system to be actually triple. From the similarity between the three spectroscopic components and the value of $\Delta m$ we assume the three components to be of very similar spectral types, and the brighter of the two visual objects to be a binary. For the distance determination, a combination of three F7/8 stars was used.

RXJ 1122.0-2446: the photometric colours give K5/7V as spectral classification. The object is a known visual binary with a separation of 0$.\!\!^{\prime\prime}$2 and a difference in visual magnitude of $\Delta m$ = 0.3, in which both visual components are binaries as well. The spectral types are (see Soderblom et al. 1996): component A: K4-5V + ??, component B: K7V + M1V. We used these spectral types to infer the distance.

RXJ 1132.9-3151: the visual magnitude being of V=3.54, this object also has photometry taken from the literature (Eggen 1977). As no Cousin's $(V-R)_{\rm C}$ and $(V-I)_{\rm C}$ colours could be found in the literature, the given indices are in the Johnson system. The distance infered from the (B-V) colour lies just in the limit, but still within the error box of the distance given by the trigonometric parallax of this star (53 pc, $\pi$ = 0$.\!\!^{\prime\prime}$019 $\pm$ 0$.\!\!^{\prime\prime}$010, Hipparcos Input Catalogue).

RXJ 1413.7-0050: the trigonometric parallax of this star gives a distance of 50 pc, with a range of 36 pc to 83 pc ($\pi$ = 0$.\!\!^{\prime\prime}$020 $\pm$ 0$.\!\!^{\prime\prime}$008, Hipparcos Input Catalogue). This star is classified as an F7Vw, so it can't be treated as a normal main-sequence star. We have therefore adopted the distance given by the trigonometric parallax to calculate its X-ray luminosity.

RXJ 1428.2-0213: with a visual magnitude of V=4.89, this object was also too bright to be observed with the 50 cm ESO. The values for the photometric colours were taken from literature (Bessel 1990). The values of the colours, as well as the measured difference in visual magnitude ($\Delta m$ = 4.2) between the two components do not agree very well with the spectral classification given in the literature (G2IV+G4V), but rather with our spectral types G6IV+K5V. The distance infered using our spectral classification (22 pc) agrees very well with the distance of 23 pc given by the trigonometric parallax ($\pi$ = 0$^{\prime\prime}$0.43, Hipparcos Input Catalogue).

RXJ 1437.5+0216: another visual binary. The separation is of 1$.\!\!^{\prime\prime}$9, and the difference in visual magnitude of $\Delta m$ = 4.0.

RXJ 1446.3+0153: V=3.72. A0/1V star. It is not very clear yet whether this is really the source of the X-ray emission ($\log (L_{\rm X}) = 28.9$) (see Sect. 3.4). The trigonometric parallax ($\pi$ = 0$.\!\!^{\prime\prime}$030 $\pm$ 0$.\!\!^{\prime\prime}$050, Hipparcos Input Catalogue) gives a distance of 33 pc, in reasonable agreement with the one calculated with the colour index.