4.2.3. Comparison of 10 Lac with stars of similar spectral type

  Another check on the results of $\S$3.1 is to cross-correlate the IUE SWP/LGAP spectra of 10 Lac with stars of similar spectral type. Such comparisons check for target-centering errors of the reference star 10 Lac as well as propagation of errors in the IUE/SWP and GHRS calibrations (i.e., a combination of errors arising from Steps 1 and 2). Two stars which provide a ready comparison with 10 Lac are HD 93521 (O9 Vp) and HD 60753 (B2 III). We were able to cross-correlate the shifts between 10 Lac and these two stars using nearly all available spectra (at least 93 exposures in each case). To do so, we again used echelle orders m = 70-119, except for m = 103, 106, and 109, which are dominated by interstellar features. Correcting for differences between the optically-determined heliocentric radial velocities for the two stars (Wilson 1953), we determined apparent radial velocity differences, in the sense ``HD-star" versus 10 Lac, of +1.8 km s^-1 ± 3 km s^-1 and -0.5 km s^-1 ± 3 km s^-1, respectively (see Table 3). The errors listed are given by assuming that the errors inherent in each of the steps described above for 10 Lac are similar for these stars and that they add in quadrature.