Conventional wisdom has it that Apple shares lose value whenever the company announces a new product, no matter how cool it is, and a 2007 study of eight events — from the 2004 Paris Expo to Macworld ’07 — by Piper Jaffray found some truth in that. The stock did fall on the day of the event four times out of eight, although on average it actually gained a fraction of a point.

But the study also suggested that investors with slightly longer perspectives could make money on Apple’s product announcements — if they knew when to place their bets. For example, Apple shares purchased one week before the event and sold one week after showed an average gain of 6.3%. And in three out of eight events the shares rose between 13.3% and 15.9%.

Does the same hold true of Apple’s annual September iTunes events?

Now that Apple has officially announced this year’s — invitations to the press were e-mailed Monday for a media event Wednesday Sept. 9 at 1 p.m. Eastern (10 a.m. Pacific) — we thought we’d do a little research of our own.

First, let’s look at that study conducted by Piper Jaffray’s Gene Munster in May 2007, just before Steve Jobs’ last keynote at the ’07 World Wide Developers Conference.

As Munster noted at the time, the rule of thumb “buy the rumor, sell the news” didn’t work as well as buying and holding. That’s because, he suggests, Apple at these events tends to deliver the goods.

Doing a similar study of Apple’s last four September events, we got a chart that looks like this:

The first thing that stands out is that 25.85% drop over a two week period last September. The market, as you may recall, was tanking at the time, thanks to Lehman Brothers’ Sept. 15 bankruptcy filing. The NASDAQ fell 12.6% between Sept. 2 and Sept. 16, 2008, and Apple, as usual, fell even harder.

But if you take the average of the last four iPod events, buying the week before and selling the week after was still a winning strategy, even with last fall’s meltdown.

And if you treat 2008 as an outlier and ignore it, it’s a strategy that showed an average gain of 6.73%.

So how cool are Apple’s announcements going to be next week?

Rumor has it that we’ll see a whole new line of iPods — some, including perhaps the iPod touch, with cameras. There’s also a chance Apple will finally announce the arrival of the Beatles catalog on iTunes, timed to coincide with Apple Corps and EMI’s release of the digitally remastered Beatles CDs on 09/09/09. We have been told, however, not to expect a tablet computer.

See also:

• How the Beatles became a video game