Panel a. Central oxygen abundance as a function of central surface brightness of the disk in the ﹩W1﹩ band. The open square marks the positon of the interacting galaxy NGC 4631 with a pure exponential disk. The open circles denote the interacting galaxies NGC 3031, NGC 3227, and NGC 5194 with broken exponential disks. The plus sign stands for the possibly interacting galaxy NGC 925 with a pure exponential disk. The points indicate other galaxies. Typical errors are shown in right lower corner. Panel b. The same as panel a but for the optical disk edge.