and @Liermann-2009. It is known that these clusters have large proper motions and do not follow closed $x_2$ orbits [@Stolte-2008; @Stolte-2014]. Stolte [et al.]{} suggested that these clusters are following transitional trajectories between $x_1$ and $x_2$ orbits. Although the proper motion is necessary to compare their 3D kinematics, [GCC-a]{} may have followed a motion similar to those of the two clusters when it was young and may not yet have settled into a circular orbit. Notably, @Lindqvist-1992 and @Deguchi-2004 reported objects with similarly large velocities, and large-velocity molecular clouds are evidenced in the CO emission in the background of Figure \[fig:LV\] [@Oka-1998]. In summary, despite the unavailability of proper motions, we can infer that the four Cepheids are rotating within the NSD, although not necessarily along closed orbits.

![image](fig7small.ps){width="0.60\hsize"}

According to our results, the Cepheids are not only currently located within the NSD but most likely formed therein. To investigate the birth sites of NSD stars, we investigated their dynamical histories in a N-body/SPH simulation of the Galaxy (Baba et al. in prep.). This simulation self-consistently accounts for the stellar dynamics, self-gravity of the gas, radiative cooling, heating by interstellar far-ultraviolet radiation, energy feedback from supernova explosions, and HII regions. 