(image source: airliners.net)
What are those humps on the rear of the wing? Are they external fuel tanks?
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For a direct comparison look at the image below. The Convair 990 has, compared to the DC-8, more windows that are more closely spaced. Also, when you look at the cockpit windows, the DC-8 has eye-brow windows while the Convair 990 lacks them. In the image you posted you can clearly see the distinct trapezoidal entry doors (wider at the top than at the bottom) of the Convair while the DC-8 has the usual rectangular doors. The most obvious difference are the "humps" on the wing of the Convair
Douglas DC-8 (left) and Convair 990 side by side (Source: Wikipedia)
The "humps", as you called them, are anti-shock bodies designed to reduce wave drag when traveling in the transsonic speed regime. The idea behind it is to keep the cross section constant over the length of the aircraft and smooth out cross section area changes. This is known as the application of the area rule.