Bottom panels: measurements of the RV of IGR 1955+0044 at three separate epochs (filled squares) with a sine fit corresponding to the f_o orbital frequency. Top panels: measurements of the continuum flux in a band around 8000 Å (open squares) and I-band magnitudes (filled dots) at the same epochs. The data are fitted with a sum of sine functions with f_s + 2(f_s−f_o) frequencies corresponding to the spin and strongest beat frequency. The shaded strips in the bottom panels denote the varying shift between phases (maximum RV vs. maximum brightness) caused by the difference of the orbital and spin frequencies.