  PN G135.9+55.9 is the most metal-poor PN known in our Galaxy. The central
star resides in a short-period binary system with a compact component, probably
a white dwarf. We describe new observations, which allowed us to determine the
orbital period. The lower limit for the combined mass of both stars is close to
the Chandrasekhar limit for white dwarfs, making this binary a possible
progenitor of a supernova type Ia. The binary system must have recently emerged
from a common envelope phase.
