Moving ghost contaminating the spectral images of OB12. Panel A: a clear frame near the mid-transit. Panel B: a contaminated frame near the ingress of the transit. Panel C: stacking all contaminated frames showing the trace of ghost. The green dashed lines indicate the waveband for white-light curves. The red arrow indicates the moving direction of the ghostfrom the redder side to the bluer side. Panel D: raw spectroscopic light curves used in Sect. 3.2. The red dots correspond to the contaminated frames, which were removed in the spectroscopic light curve fitting. We note that the horizontal moire fringes shown in Panel A, B, and C were caused by the 500-kHz fast readout mode adopted in OB12 and were visually magnified due to a small figure size, which barely affected the stellar fluxes.