Since we adopted $55~{{\mathrm{km~s}^{-1}}}$, this range introduces an error of $\pm 13~{{\mathrm{km~s}^{-1}}}$ in mean velocity, because the mean estimate is affected by half the change of the velocity amplitude if a measured velocity is obtained at the extreme.

Discussion
==========

Figure \[fig:LV\] plots the $V_\mathrm{LSR}$ of the Cepheids against the Galactic longitude. The longitude and $V_\mathrm{LSR}$ of [GCC-a]{} are both positive, whereas those of [GCC-b]{}, -c, and -d are negative. This result is consistent with the prediction that the Cepheids orbit around Sgr A$^*$ similar to other objects in the NSD. The solid curves in Figure \[fig:LV\] are extracted from Figure 1 in @Stark-2004 and indicate several $x_1$ and $x_2$ orbits from the model of @Bissantz-2003. Their model adopts the bar gravitational potential constructed from photometric data collected by the [*COBE*]{} satellite [@Bissantz-2002].

Examining the $l$-$v$ diagram, we find that [GCC-b]{} and -c follow inner $x_2$ orbits, whereas [GCC-d]{} follows an outer $x_2$ orbit. [GCC-b]{} and -c are close to each other not only in projected position but also in radial velocity. The error of their estimated distances is $\pm 500$ pc, which does not allow us to determine their line-of-sight positions within the NSD (radius $\sim 200$ pc). It 