Based on RATAN-600 21 cm line observations with an angular resolution of 2.4' over a wide range of radial velocities, we analyze the neutral-hydrogen distribution in the region of the supernova remnant (SNR) G78.2+2.1.
In addition to an HI shell at low radial velocities immediately surrounding the radio remnant, we detected an extended expanding HI shell, 3 dg. in diameter, at a radial velocity of -25 km/s, which closely coincides in coordinates and angular sizes with the outer X-ray shell discovered by Lozinskaya et al. (2000).
The H_α emission studied by these authors in the SNR region also has a secondary peak at radial velocities from -45 to -20 km/s. Since the radial velocities of these two objects differ markedly, their distances can be assumed to differ as well; i.e., a chance projection of two distinct objects is observed.

Figure 1. The brightness distribution of H I features at a radial velocity of +10 km/s after an extended background in the region of the Cygnus X radio source and SNR G78.2+2.1 has bee subtracted. The declinations of HI cuts are indicated on the right; the vertical and horizontal scales are different. The thin lines represent the annular H I structures related to the above objects (Gosachinskii et al. 1999).

Figure 2. A schematic image of SNR G78.2+2.1 as constructed from the 21cm data of Higgs et al. (1977) (heavy circle) and X-ray (0.5-2.0 keV) isophotes from Lozinskaya et al. (2000). The vertical and horizontal scales and approximately the same.

Figure 3. Same as Fig. 1 at radial velocities of -20, -25 and -30 km/s. The thin lines represent the cavity that coincides in coordinates with the outer X-ray shell found by Lozinskaya et al. (2000) and shown in Figure 2.

Figure 4. The distribution of HI-line brightness temperature in the vicinity of G78.2+2.1 as constructed from data of the survey by Westerhout and Wendlandt (1982) with an angular resolution of 12' and a radial-velocity resolution of 2 km/s1. The source center (heavy lines) and the sizes at "zero" brightness temperature as inferred by Reich et al. (1990) at 11 cm are indicated.