All brandy and Cognac are 100% low carb - they have zero carbs in them. Brandy and Cognac are both styles of distilled wine - i.e. made from grape product. You might ask why something made from grapes can have zero carbs. The answer is in how brandy and Cognac are made.
First off, Cognac is simply brandy. It's Brandy that is made in the Cognac region of France, up over Bordeaux.
Brandy was created for the British. The British were shipping lots of wine from Portugal up to England - and the wine took up a lot of space in the ship's hold. To save space, they boiled off the water at the dock, and just added water back in at the receiving side. Soon they found that they liked the concentrated flavor of the distilled version better.
Armagnac is another style of Brandy, that comes from Gascony, France.
The process of making wine is that yeast eat up the sugars in the grapes and turn it into alcohol and carbon dioxide. With brandy, by the time it is distilled and fermented, there is pretty much zero sugar left. That makes it a great low carb item!
Fruit Flavored Brandies
There are certainly liqueurs where they have added sugars and flavors into brandy, creating a concoction that has carbs. So for example Hiram Walker Peach Brandy. These would definitely have carbs in them.
I have written in to Hiram Walker to determine how many carbs, and will post here once I hear back from them.
Alcohol Carb Charts
Lisa Shea's Library of Low Carb Books