Pollen does sometimes get transferred by insects however. So if you want to be certain, bag the blossoms with a little bag made of tulle. If you are willing to take the slight risk of cross pollination, just separate them from other tomatoes by planting them on opposite sides of some other crop.

Jim

Your heirloom is "dehybridized," and as such, will render seeds that are true to the strain, if not X-pollinated. So we agree on that.

Your intuitive comments are 100% correct. Absolutely those hybrid tomatoes can provide pollen that might get to your heirlooms, and that would ruin your seed stock...but would have no effect on the taste or nature of the tomatoes coming from your heirloom vine. Only the SEEDS are affected. (Just thought I'd throw that in, as the misconception has been voiced before.) And, given your astute intuition, you should see the logic of this. While the pollination gets the fruit to set and develop, the genetic specifics are relevant only to the resulting seeds, not flesh of the fruit.

Of course, the fruit of the subsequent generation would be entirely affected by the genetic nature of the plant that produces the fruit.

But, back to your original inquiry...

As your plants develop, I suggest you closely examine the flowers of your plants. You should notice that your hybrids typically have flowers where the male part (anthers) completely envelope the female part (pistil and stigma). This virtually assures self-pollination, and pollination at a high rate. Hence the prolific yield of the typical Early Girl or Better Boy.

The heirloom, ironically, has a long female part (pistil) which tends to encourage cross-pollination, so anatomy is working against your desire to favor self-pollination. (I suppose the term "OP" (Open Pollination) comes from this anatomical aspect, but I am only surmising that...no confirmation from authoritative writings.) So, once again, common sense should be considered. And, as expected, heirlooms tend to have low yields...in my opinion, because the anthers of these flowers are often unable to "get to" the stigmas, which ride high and above the anthers.

So I manually pollinate them.

But I never collect my own seeds this way. I just want to eat the fruit!

Again, trust your senses. If pollen from a different tomato variety might pollinate your heirloom, then the seed is junk. And I suppose that pollen from a different plant that is the SAME varietal should be OK...but I have never read that anywhere.

I suppose I reserve the right to trust my common sense, too.

If the heirloom is irreplaceable, however, "reasonably pure" may not be good enough. In that case, the blossom bagging suggested by several posters is the best option.