Periodograms of residual velocity variations in targets HD 9312, HD 9939, HD 11613, HD 14214, HD 15814, and HD 35956. The horizontal axis represents the period. The vertical axis represents the power. The dotted horizontal lines represent the FAP levels of 0.10|$\%$|⁠, 1.00|$\%$|⁠, and 10.00|$\%$|⁠, from top to bottom. The inverted triangles show the main strong peak. The period showing the strongest peak is regarded as the period of the residual-velocity variations. (Color online)