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tokens: to orbital phase 0.9364, close to the primary eclipse, for which the contribution of the secondary is minimum. Figure 5. Same as Fig. 4 , but for one of the MagE spectra corresponding to orbital phase 0.9352. The results of the quantitative spectral analysis of all ALFOSC and MagE spectra are summarized in Fig. 6 . In the left-hand panels (ALFOSC), the apparent variations of the effective temperature with phase and an amplitude of ∼2500 K are obvious. The lowest temperatures (∼40 kK) occur near primary eclipse and in the secondary eclipse, where the contribution by extra light should be minimal. Hence, the increase during other orbital phases is caused by reflected light and, therefore, not real. Similarly, variations of the helium abundance are observed. The apparent variations of the surface gravity, however, are small. The mean values adopted for T eff , log g , and log y , summarized in Table 2 , are obtained by selecting six spectra close to the primary eclipse with phase between −0.1 and +0.1 and adding one spectrum at phase 0.52. The analysis of the MagE spectra also results in effective temperatures and helium abundances that seem to vary with orbital phase (cf. Fig. 6 , right-hand panels), but with amplitudes less pronounced than those from the ALFOSC spectra. For this reason, we use all the MagE spectra to compute mean values and standard deviations of T eff and log y (cf. Table 2 ). Figure 6. T eff , log g , log y , and v rot sin i variations as a function of the orbital phase. Left-hand panels: ALFOSC. The green dotted–dashed horizontal lines mark the adopted T eff , log g , and log y and the associated errors (cf. Table 2 and text). Note the two points with low T eff near phase 0.5 (secondary eclipse), when the contribution of the secondary is strongly reduced. Right-hand panels: MagE. The orange dotted–dashed horizontal lines mark the average values and associated errors of T eff , v rot sin i , and He abundance. In the central panel, note the outlier near phase zero (primary eclipse). Table 2. SdOB atmospheric parameters and rotational velocity. The effective temperature and surface gravity of EPIC 216747137 ( T eff = 40400 K and log g = 5.56) are very similar to the hot HW Vir systems AA Dor (Klepp Rauch 2011 ) and V1828 Aql (= NSVS 14256825, Almeida et al. 2012 ). EPIC 216747137 also shares an underabundance of helium (log y = −2.59) with the two others. The projected rotational velocity, as derived from the individual MagE spectra (central right panel of Fig. 6 ), results in a mean v sin i = 51 ± 10 km s −1 , significantly less than ∼70 km s −1 expected for tidally locked rotation. Mean rotational velocity of 51 km s −1 and standard deviation of 10 km s −1 are obtained excluding only a single outlier close to phase 0 (see central right panel of Fig. 6 ). 5 STELLAR PARAMETERS: RADIUS, MASS, AND LUMINOSITY The second data release of Gaia provided a precise (5 per cent) parallax measurement which allows the stellar