Absolutely speaking, the binary is effectively cooler than our Sun. The primary has a mass of 3/5 solar masses, with the corresponding main-sequence spectral class (K) and radius (~0.7 solar radii), and thus luminosity, which works out to be about 10% of the solar luminosity. The remainder of the mass is, as I said, locked into a compact object, specifically a medium-cool (spectral class DC) White Dwarf, which is bright but so small that it doesn't make any noticeable contribution.
That said, the binary as it appears in the planet's sky would indeed be quite a bit more intense than our Sun appears in Earth's, for a period from a few days before to a few days after perihelion. At Earth's temperate latitudes, the change in insolation between summer and winter is a factor of a little below 10 (yes, it really is that much). The theoretical difference in seasonal equilibrium temperatures due to that change should be on the order of 50% (150 degrees Kelvin), but the combined dampening effect of heat buffering and heat transfer reduces that to an actual difference of a mere 10% (30 K).
On my planet, the change in insolation between perihelion and aphelion is a little below 100. The theoretical temperature variation would be on the order of 100% (300 K). I estimate the dampening effect to be about the same as it is for Earth, since the buffers are essentially similar, and the transfer within each hemisphere somewhat more efficient due to a more dominant Hadley Cell
, which compensates for the absence of any transfer between hemispheres due to the seasons' being global instead of hemispherical as on Earth. So, everything else being equal, a good guess at the actual temperature changes over the course of the year would be 20% (60 K).
However, all else is not equal - the year is only a quarter as long. On Earth, the seasons lag behind the extremes in insolation by about six weeks, which is why we use the solstices to mark the start, rather than the middle, of summer and winter. On my planet, perihelion and aphelion itself are only six Earth-weeks apart. Consequently, that 20% temperature variation never gets a chance to fully materialize. The perihelial phase of the orbit just doesn't last long enough for the buffers to fill up all the way, which depresses the summer temperatures further, and similarly the aphelial phase of the orbit just doesn't last long enough for the buffers to empty all the way, which keeps the winter temperaturs from dropping as far as they otherwise might. So, 15% variation should be closer to the mark. I've actually done some (admittedly extremely rough) numerical modelling, and these are the seasonal temperatures projected by that model, calibrated to an annual gobal mean of 15 degree Celsius, just as on Earth:
|Lat 00||50.0 C||39.9 C||27.7 C||16.2 C||7.2 C||16.5 C|
|Lat 15||48.6 C||38.5 C||26.4 C||15.0 C||6.0 C||15.2 C|
|Lat 30||44.3 C||34.3 C||22.3 C||11.1 C||2.2 C||11.3 C|
|Lat 45||36.3 C||26.6 C||14.9 C||4.0 C||-4.7 C||4.2 C|
|Lat 60||23.2 C||13.9 C||2.7 C||-7.8 C||-16.1 C||-7.6 C|
|Lat 75||-0.2 C||-8.8 C||-19.1 C||-28.7 C||-36.4 C||-28.5 C|
The biggest difference to Earth is that equatorial and tropical summers are a bit heftier than those on Earth. Essentially, this compensates for the colder poles, which unlike Earth's never get any direct sunlight to speak of whatsoever (then again, the sky never gets dark at all, either).
So, no liquefaction of the atmosphere in the offing, I'm afraid. Does that put your concerns to rest, for the most part?
As to the why, I decided on a whim that a different kind of seasons might be interesting, with "interesting" turning to "awesome" if as a side-effect there is a visible change to the appearance of the sun(s) in the sky (to their sizes, in this case), as opposed to just to their paths across it. I wasn't all that confident that this would be viable at first myself, but eventually came across this article
which fairly decisively establishes that it indeed is.
To my surprise, the biggest challenge my planet seems to face is actually due to the tidal effects exerted by the binary around perihelion, which can grow to almost one thousand times the magnitude of those our Moon exerts on Earth. Permanent settlements below 1 km above sea-level don't seem feasible at all, as they'd just be periodically deluged by the water tides, and the effects of the air tides (significant pressure changes) and ground tides (quakes) need to be carefully taken into account. If, that is, I want to keep this all the way on the hard side of the scale. I daresay most readers don't really think about ground tides and have never even heard of air tides, so I ought to be able to get away with quite a bit of fudging in those regards without making things seems any less plausible.
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.