($\equiv$ HD157406, WC7 + O7-9, @vanderhucht01) is also projected onto this region. Massey et al. (2001) consider this star to be a probable member of the cluster. However, the large uncertainties in reddening and distance determinations cast doubts on its relation to the cluster. In addition to the visible stars, massive OB stars hidden in areas with high reddening may also contribute to the ionization and photodissociation of the gas [@felli90].

The study by @persi86 of this region focussed on stellar formation activity. This study, along with more recent searches for YSOs in the region, resulted in the discovery of a large number of infrared and X-ray source candidates to YSOs and massive stars with high reddening [e.g., @bohigas04; @wang07], showing that the whole  complex is an active area of recent and on-going star formation.

Distance determinations to the ionized complex span the range 1.7 to 2.6 kpc. @neckel78 and @lortet84 derived a distance of 1.7 kpc from UBV and H$\beta$ data, and from the nebular reddening, respectively. @massey01, based on recent spectral classification and absolute magnitude calibration, redetermined the distance to Pismis 24, adopting $(m-M)_o$ = 12.0 mag, or $d$ = 2.5 kpc. Following @massey01, we adopt a distance 