diff --git a/pyerrors/pyerrors.py b/pyerrors/pyerrors.py
index 0e8bfeb4..f5e58494 100644
--- a/pyerrors/pyerrors.py
+++ b/pyerrors/pyerrors.py
@@ -641,7 +641,9 @@ class Obs:
 
 
 class CObs:
+    """Class for a complex valued observable."""
     __slots__ = ['real', 'imag', 'tag']
+
     def __init__(self, real, imag=0.0):
         self.real = real
         self.imag = imag