spectrum is compared with the observed spectrum (4). The lower panel shows the difference between the spectra (3) and (4). \[fig:comp\]](fig3.ps){width="0.98\hsize"}

Cepheid pulsations can alter the radial velocities measured at each epoch by tens of ${{\mathrm{km~s}^{-1}}}$ [e.g., @Pejcha-2012]. First of all, we need the pulsational phase in each spectrum to discuss such variations. The pulsational phase is usually defined based on the optical $V$-band curves, but it is not possible to obtain the optical light curves of our targets because of severe extinction. Here, we define the zero phase at the $H$-band minimum. For our purpose, we require only the relative phase offsets between the $H$-band light curves and the radial velocity curves.

To obtain mean velocities of the target Cepheid, we first constructed a template of the velocity variations of the Cepheids with $P\simeq 20$ days. All our targets have similar periods, but the prediction template is yet to be established. We adopted the infrared photometry and radial velocities of 11 nearby Cepheids with $18 \leq P \leq 24$ days (Table \[tab:templates\]) compiled by @Groenewegen-2013. First, the $H$-band and velocity curves of each template Cepheid were fitted by seventh-order Fourier series, and the amplitude and the mean were obtained. 