Longest sequence of primes where each term is obtained by appending a new digit to the previous term What is the longest known sequence of primes where each new term is obtained by appending a new decimal digit to the previous term?
Examples:
$$(2,23,233,2333,23333)$$
There are no more members in this sequence, because $233331,233333,233337,233339$ are all composite numbes.
$$(3,37,373,3733,37337,373379,3733799,37337999)$$
This one is longer, but still only $8$ terms.
An so on. Because prime gaps can become arbitrary long for large numbers, there is no way any such sequence will be infinite. On the other hand, they can be very long, I think.

What is the longest known sequence of this kind?
Can we prove that:

*

*There is no infinite sequence of this kind. Or at least that the sum of reciprocals of any such sequence is finite.


Of course, we can study such sequences in binary or other bases.
Edit
The sequence can start with any prime. This link is very relevant. See also the comments.
The argument that an infinite sequence like this is unlikely goes like this:
For each new term we have only four possibilities:
$$a_{n+1}=10 a_n+b,~~~~b=(1,3,7,9)$$
All of these numbers are contained in a very small interval, about $10$ in size. But the density of primes becomes smaller with growing number of digits, and the gaps become larger. In my opinion, the probability of finding the next term for any such sequence becomes smaller with growing $n$.
It doesn't mean that an infinite sequence is impossible, but I conjecture that it's impossible to prove that any such infinite sequence exists.

This question seems to be very closely related to an older question and a more recent question

It appears such a sequence cannot be infinite. Basically, what you're doing is multiplying by $10$ and adding either $1$, $3$, $7$ or $9$. I wrote some python code to test whether sequences of a certain length existed. 
I started with a list of all the integers from $0$ up until the highest prime below $10000$ minus one. These are all the possible values of $n\ mod\ p$ with $0<p<10000$. Then I made a new list, which contained all the the values in the old one, multiplied by $10$ and with $1$, $3$, $7$ or $9$ added to them. I removed all the multiples of primes (not the primes themselves) and repeated this proces. As it turns out, the list is empty after repeating this proces $15$ times
Code:
>>> L = []
>>> for x in range(max(primes)):
    L.append(x)


>>> for x in range(15):
    L2 = []
    for l in L:
        L2.append(10*l+1)
        L2.append(10*l+3)
        L2.append(10*l+7)
        L2.append(10*l+9)
    L3 = []
    for l in L2:
        zero = False
        for p in primes:
            if l%p==0 and l!=p:
                zero = True
                break
        if not zero:
            L3.append(l)
    L = copy(L3)


>>> len(L)
0
>>> 

This means that no sequences of length $19$(the initial $4$-digit number can also be a prime with this very special property) exist. As a matter of fact, there don't even exist sequences of length $18$.
