Top: Residuals after subtraction of the disk model from the Q_ϕ image of WRAY 15-1880 on the disk plane. A high-pass filtering has been applied by subtracting the median over 7 pixels, to enhance structures. The red circle marks the star position; the magenta circle marks the position of the candidate planet at the epoch of this observation. The dotted magenta line is a possible circular orbit for this planet. The red solid lines mark possible spiral arms around the star. They are at 140 degree from each other. Bottom: Peak of the possible spiral arm feature as a function of the azimuth angle ϕ with respect the star on the disk plane (asterisks). ϕ is defined from the northern position of the major axis toward the east. The open diamond marks the planet position.