tokens: are sub-Keplerian by ≈20%, in good agreement with other studies (e.g., Wade Horne 1988 ). Note that for q ≳ 0.25, the 3:1 resonance lies beyond ≈0.9 R L 1 and the outer disk radius is then limited by the largest non-intersecting orbits allowed by three-body interactions (Paczyński 1977 ). Under these circumstances the dependence of the outer disk radius on q is very weak (Frank et al. 2002 ) and therefore a nearly constant evolution of DP/FWHM versus q is expected for q ≳ 0.25. For practical purposes, in Figure 3 we display the variation of DP/FWHM versus q in logarithmic units. A least-squares linear fit yieldswith a Pearson correlation coefficient r = 0.95. To estimate the error in q implied by this relation, we computed the difference with respect to the true observed values for our 11 SXTs. The distribution of differences can be approximated by a normal function with σ ( q ) = 0.015. This indicates that mass ratios can be realistically obtained from Equation ( 6 ) with a typical ∼25% uncertainty. Figure 3. Correlation between q and the parameter DP/FWHM in log units and the best linear fit. 4. ORBITAL EFFECTS The spectra that we used to produce the DP/FWHM– q correlation are orbital averages. This is party because individual spectra rarely possess enough signal-to-noise for the technique to be applicable. But also because orbital means average out possible asymmetries in individual spectra from, for example, hot spots or disk eccentricities that could potentially bias the determination of q . At this point we decided to explore the impact of line asymmetries in the results of our technique. Since this can only be tested on data with sufficient signal-to-noise, we have focused on the 154 high-quality GTC spectra of XTE J1118+480 obtained along four different orbits over two years. We have performed Gaussian fits on every individual spectrum and computed mass ratios using Equation ( 6 ). The distribution of q values is found to peak at q = 0.024 (the bottom panel in Figure 4 ), with 68% of the values contained between q = 0.021 and 0.045. This indicates that if q were to be obtained from a single individual spectrum, the typical uncertainty would be about ∼56%. Figure 4. Bottom: histogram of q values derived through Equation ( 6 ) from 154 GTC spectra of XTE J1118+480. Top: variation q with the V / R parameter of the H α profile (see the text for details). In order to trace the effect of line asymmetries we also extracted the V / R parameter from each spectrum, with V / R defined as the ratio of equivalent widths between the blue and the red part of the H α profile. The two halves of the line are set from the rest wavelength till ±2500 km s −1 . For example, indicates a line with the red part stronger by 20% while implies a symmetric profile. A plot of q versus V / R (see top panel in Figure 4 ) seems to show a trend, with asymmetric profiles preferring slightly lower q values, although the large scatter prevents from drawing a firm
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