Puppis clusters Haffner 19 and 18ab and the 15 kiloparsec arm

Photoelectric and photographic photometry is presented for 584 stars in the Puppis open clusters Haffner 19 and l8abc and in the surrounding field (total area 0.063 sq deg). The clusters are located 6.9i0.9 kpc from the Sun in the direction 1 = 243?1, b = +0?S. Haffner 19 (earliest observed spectral type BO) is radially symmetric; Haffner 1 8ab (07) is an elongated cluster parallel to the galactic plane; Haffner 1 8c is probably not a real cluster. The surrounding region appears to contain many faint 0 and B stars at about 8i 1 kpc from the Sun; the approximate space densities are 25 X for 0 to b 2.5 stars and the same for b 3 to b 6 stars. These densities are typical of an arm region, but much higher than found between 1 and 6 kpc, indicating the presence of a major spiral feature coincident with the HI Perseus extension (Davies 1972). The field stars, reddened by Ey =0.5 to 1.3, appear to be embedded in dust associated with the arm. The stars of Haffner 19 and 18ab are also reddened by varying amounts; however, their mean color excesses, 0.45 and 0.70, respectively, are less than the average of the surrounding region (0.90), indicating that the clusters probably lie on the inner side of the 15 kpc spiral arm. The kinematic distances of Haffner 19 and 1 8ab (and several H II regions at 230<1 <245: Georgelin et al. 1973) are substantially less than the photometric distances.