Only the argument with one decimal place precision does NOT overflow.

Python 2.7a1+ (trunk:76872, Dec 21 2009, 09:54:29) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import expm1
>>> expm1(709.78271289338397)     
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: math range error
>>> expm1(709.782712893)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: math range error
>>> expm1(709.7827)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: math range error
>>> expm1(709.782)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: math range error
>>> expm1(709.78)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: math range error
>>> expm1(709.7)
1.6549840276802644e+308