The H ii regions Sh 217 and Sh 219 are textbook examples of a Strömgren sphere surrounded by an annular photodissociation region ( PDR ) .
The annular PDR is observed in both the 21 cm atomic hydrogen emission and the dust ( PAH ) emission near 8 \mu m ( MSX Survey ) .

An ultracompact radio continuum source is observed in the direction of the annular PDR , in both Sh 217 and Sh 219 . JHK observations show the presence of highly reddened stellar clusters ( A _ { V } \sim 20 mag ) in the directions of these radio sources .
These clusters are also IRAS sources , of luminosities 22 700 L _ { \odot } for Sh 217 and 5900 L _ { \odot } for Sh 219 .
Each cluster contains at least one luminous star with an IR colour excess ; the one in the Sh 219 cluster shows H \alpha emission .
The cluster associated with Sh 217 is almost spherical and contains luminous objects at its centre .
The cluster associated with Sh 219 is elongated along the ionization front of this H ii region .

We argue that these are ‘ second-generation clusters ’ , which means that the physical conditions present in the PDRs , close to the ionization fronts , have favoured the formation of clusters containing massive objects .
We discuss the physical mechanisms which may be at the origin of the observed triggered star formation .