The population of high‐energy and very high‐energy γ‐ray sources, detected with EGRET and the new generation of ground‐based Cherenkov telescopes, forms a reduced but physically important sample. Most of these sources are extragalactic (e.g. blazars), while among the galactic ones there are pulsars and supernova remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al., has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high‐energy γ‐ray sources. To model and understand how the energetic photons are produced and escape from LS 5039, it is crucial to unveil the nature of the compact object, which remains unknown. Here, we present new intermediate‐dispersion spectroscopy of this source, which, combined with values reported in the literature, provides an orbital period of Porb= 3.906 03 ± 0.000 17 d, a mass function f(M) = 0.0053 ± 0.0009 M⊙ and an eccentricity e= 0.35 ± 0.04. Atmosphere model fitting to the spectrum of the optical companion, together with our new distance estimate of d= 2.5 ± 0.1 kpc, yields RO= 9.3+0.7−0.6 R⊙, log (LO/ L⊙) = 5.26 ± 0.06 and MO= 22.9+3.4−2.9 M⊙. These, combined with our dynamical solution and the assumption of pseudo‐synchronization, yield an inclination and a compact object mass MX= 3.7+1.3−1.0 M⊙. This is above neutron star masses for most of the standard equations of state and, therefore, we propose that the compact object in LS 5039 is a black hole. We finally discuss the implications of our orbital solution and new parameters of the binary system on the CNO products, the accretion/ejection energetic balance, the supernova explosion scenario and the behaviour of the very high‐energy γ‐ray emission with the new orbital period.

We discuss the properties of 137 cataclysmic variables (CVs) which are included in the Sloan Digital Sky Survey (SDSS) spectroscopic data base, and for which accurate orbital periods have been measured. 92 of these systems are new discoveries from SDSS and were followed‐up in more detail over the past few years. 45 systems were previously identified as CVs because of the detection of optical outbursts and/or X‐ray emission, and subsequently re‐identified from the SDSS spectroscopy. The period distribution of the SDSS CVs differs dramatically from that of all the previously known CVs, in particular it contains a significant accumulation of systems in the orbital period range 80–86 min. We identify this feature as the elusive ‘period minimum spike’ predicted by CV population models, which resolves a long‐standing discrepancy between compact binary evolution theory and observations. We show that this spike is almost entirely due to the large number of CVs with very low accretion activity identified by SDSS. The optical spectra of these systems are dominated by emission from the white dwarf photosphere, and display little or no spectroscopic signature from the donor stars, suggesting very low mass companion stars. We determine the average absolute magnitude of these low‐luminosity CVs at the period minimum to be 〈Mg〉= 11.6 ± 0.7. Comparison of the SDSS CV sample to the CVs found in the Hamburg Quasar Survey and the Palomar Green Survey suggests that the depth of SDSS is the key ingredient resulting in the discovery of a large number of intrinsically faint short‐period systems.

Aims. During the last ∼50 years, the population of black hole candidates in X-ray binaries has increased considerably, with 59 Galactic objects being detected in transient low-mass X-ray binaries, as well as a few in persistent systems (including ∼5 extragalactic binaries). Methods. We collect near-infrared, optical, and X-ray information spread over hundreds of references to study the population of black holes in X-ray transients as a whole.Results. We present the most updated catalogue of black hole transients. This contains X-ray, optical, and near-infrared observations, together with their astrometric and dynamical properties. The catalogue provides new and useful information in both statistical and observational parameters and provides a thorough and complete overview of the black hole population in the Milky Way. Analysing the distances and spatial distribution of the observed systems, we estimate a total population of ∼1300 Galactic black hole transients. This means that we have only discovered less than ∼5% of the total Galactic distribution.

New optical spectroscopy of the high‐mass X‐ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X‐ray/radio outbursts are triggered 2.5–4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ∼65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i≃ 113 km s−1.